ISO Invariance

Hi everyone....I have been mulling over some tech stuff for actually about a year+ (quantum efficiencies, differences in sensors, how much head room really exists over my Canon 5DIV, etc).

But as I am working on my thoughts which include how ISO works, Tony Northup came along and stirred up the "ISO Invariance" pot. I am not sure he even knew what ISO Invariance was when he did his first video, as he just said "ISO is Fake." Fstoppers then did their own videos, and everyone seemed to finally notice iso invariance even though people have been talking about it for 2-3 years and I think DPReview has had a test for it for ~2 years.

If you are not familiar, ISO Invariance in short is the fact with modern cameras adding "gain" (increasing exposure, etc) in post processing is almost identical to increasing ISO while shooting. And if you watch Tony's or Fstoppers videos, you'll see them underexpose images by 4-5 stops, lift them in post, and have the noise be almost identical to a shot that was properly exposed in camera. Note, this is really about under exposing, as a blown highlight cannot be recovered and, not mentioned in the videos, there are still numerous benefits to properly exposing such as working with flashes and cutting down post processing, and, in my experience, I find some images are just best when dialed in in-camera (colors, feel, etc).

But that said, the natural conclusion to ISO invariance is that you can under expose certain modern cameras with little penalty, and I do use this in my photography and this is one of the reasons I bought the 5DIV (that and f/8 AF points, etc). For high contrast scenes with a bright subject (loons, sunrises, etc) I often expose for the brighter part of the subject, underexpose the shadows, and bring them up in post. While ETTR still exists, it is less important (IMO) than it was 5-10 years ago.

But...all of this messed with my understanding of how ISO actually physically works. It is a bit funny that one thread is talking about old active users, as I am pretty sure my understanding of ISO and how it works came from Daniel Browning years ago. Maybe Neuro, or a combination.

My current understanding, camera sensors are charged diodes, and there are these basic steps:

1. The camera diode/sensor is charged
2. The charged diode releases electrons when hit by light
3. The released electrons are then captured by the pixel
   (electrons captured per photon of light is quantum efficiency)
4. In a CMOS sensor, the electrons are read by a transistor at the pixel level which creates an analog signal
5. Gain can be added to an analog signal
6. The analog signal is then converted to a digital signal
7. The digital signal is recorded as part of the RAW file.
8. Gain can be added in post processing

Some key things missing in the above that is relevant for some sensor discussions includes how the analog signal is read (in rows = rolling shutter; all at once = global shutter with obvious implications to on sensor circuitry/bandwidth) and where the analog signal is converted to digital (off sensor is the old method with more noise while on sensor has less noise as the signal does not have to travel as far).

So, with the above description, the question is, what is "ISO"?

Based on the TN and Fstopper videos and some online searches, what is being implied is that in camera ISO is simply an amplification to the analog signal in step 5. Whereas adding gain in post processing is simply adding to the digital signal in step 8. But Steps 1-4 never change while taking a picture.

Under this thinking, ISO invariance exists now because read/dark noise decreased to the point of being negligible when moving the analog/digital conversion on chip. In other words, since the digital signal is essentially as clean as the analog signal, it no longer matters very much where you add gain: in camera or in post processing.

Seems reasonable, I can find charts where that seems to make sense (DR/noise is linear with increasing ISO, etc).

But, it flies in the face of my understanding of what ISO is and I can also find charts where DR/noise are not linear with increasing ISO.

Quickly, my understanding of ISO was two things:

1. The camera diode/sensor received different charges at certain key ISOs, so the charge in step 1 changes at certain ISOs
2. Then the camera would add gain or subtract gain from the analog signal in this push/pull method that created this wavy DR/Noise graphs (step 5). Even today with a modern sensor like the A7RIII, there are two points from which it is linear, not one, implying two "base" ISOs.

But, I can't find anyone referencing variations in diode/sensor charge as part of ISO (variations to Step 1). It is all analog signal gain, or step 5 above.

So, after all that, the question is, does anyone know or know of a good reference for what ISO actually is? BTW, feel free to point out inaccuracies in the above. I am a Civil/Environmental Engineer, not an Electrical.

Thanks,
Brant

BTW, if you want examples:

Wavy results which to me imply either a variation in charge on the diode (step 1), or I actually see reference to a "dual gain" as explanation for the wavy behavior (so, a double step 5? or gain to analog and then gain to the digital signal?)
Attachment 2769
The blue (5DIII) line shows the wavy behavior while the black (5DIV) sensor having a little, but a lot less waviness and much straighter (iso invariant).

Then Sony sensors have straighter lines (which I can see being gain added at a step 5), but the A7RIII this one drop (which I suspect is something else and have previously thought it was a charge difference at step 1):
Attachment 2770

If interested, as Nikon buys their sensors, the behavior of their sensors varies with the source. But you can see wavy behavior, or straight line with one drop.

Interesting review... I'm fascinated by this stuff although most of it is way over my head (mechanical engineer, barely passed my mandatory electrical course 30+ years ago). I'd like to know where noise comes into play in all this. What is the intrinsic flaw (if that's the right word) in the electronics that creates noise? Do the diodes become less accurate as charge increases with ISO?

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Originally Posted by Kayaker72

But that said, the natural conclusion to ISO invariance is that you can under expose certain modern cameras with little penalty, and I do use this in my photography and this is one of the reasons I bought the 5DIV (that and f/8 AF points, etc).

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So wouldn't this make sense?

If your camera is set on the exact same shutter speed and aperture the sensor gathers the exact same amount of data regardless of the ISO setting.
If you adjust your picture to the proper exposure in post the data was always there, you would only be changing the point in the process it is done.

Quote:

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Originally Posted by Kayaker72

1. The camera diode/sensor received different charges at certain key ISOs, so the charge in step 1 changes at certain ISOs

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I have never heard that one, why would it?

I do remember old conversations or debates about noise. A certain amount of noise is introduced at the sensor, additional (spatial) noise when you increase gain.
So perhaps if someone could explain how increasing the gain works, does it change the signal to noise ratio?

Engineers::confused:
https://www.sis.se/api/document/preview/907324/
https://webstore.ansi.org/Standards/...SAAEgLtPPD_BwE

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Originally Posted by Jonathan Huyer

Interesting review... I'm fascinated by this stuff although most of it is way over my head (mechanical engineer, barely passed my mandatory electrical course 30+ years ago). I'd like to know where noise comes into play in all this. What is the intrinsic flaw (if that's the right word) in the electronics that creates noise? Do the diodes become less accurate as charge increases with ISO?

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Yeah....this Civil Engineer is breathing through a straw on a lot of this.

First, I am becoming convinced that I either misunderstood or whoever suggested ISO was based on varied diode charge was wrong. To me, it does make sense as increase charge would be more sensitive (fewer photons in low light but more electrons to get bumped off the sensor), but also more potential for noise (random electrons getting bumped off, more heat, etc). But, I even came across this reference where myth #1 on clarkvision states that "ISO changes sensitivity" is false. Then, i

n all the articles I have read that actually look this granular (which actually isn't many), I only see references to ISO being adding gain to the analog signal and that the diode/sensor to pixel/photon wells are the same for every ISO setting (i.e. base ISO). So, as the analog digital conversion has become more efficient (granted, only for some sensors), you get ISO Invariance.

Overall, it sounds like the "myth" of changing sensor sensitivity has been out there, I picked it up someplace.

It looks like the article I like for noise is no longer on the web (at least my link doesn't work). But in terms of understanding what is happening on the sensor level, this video is great (also talks about film, which is fun):

https://www.youtube.com/watch?v=MytCfECfqWc

Then I also like this article:

http://dpanswers.com/content/tech_iso.php (old...it is actually one of my first saved links for photography)

I came across this article while checking my understanding of ISO and ISO-Invariance. But it has a nice discussion of noise part of the way down:
https://photographylife.com/iso-invariance-explained

But, generally, there is noise associated with the variation of light (i.e. shot noise) that has nothing to do with our cameras, but then there is noise introduced during exposure, with inaccuracies in the voltage readout, while the analog signal is transferred (this is what plagued/plagues Canon), and then during the analog to digital conversion (again, any inaccuracies, electrons being added subtracted, etc). Overall, anything within the camera is generally termed "read" noise.

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Originally Posted by HDNitehawk

So wouldn't this make sense?

If your camera is set on the exact same shutter speed and aperture the sensor gathers the exact same amount of data regardless of the ISO setting.
If you adjust your picture to the proper exposure in post the data was always there, you would only be changing the point in the process it is done.

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Yes, this makes sense for a truly ISO invariant system. However, in older systems and some modern systems, they are not iso invariant. For example, with ISO being added to the analog signal, all noise that happens while generating the analog signal is also increased. But all noise added while transferring the analog signal (in some cases, off chip) and while converting the analog to a digital signal is not amplified. Thus, with the exact same exposure settings you would get less noise in your RAW file if proper exposure was ISO 800 by setting ISO in camera to ISO 800 (adding gain to the analog signal) rather than shooting at ISO 100 (base ISO for most cameras) and adding 3 stops of gain post digitization/creation of the raw file. ISO invariance, the noise added after creating of the analog signal is negligible, so you could shoot ISO 100 and increase in post and have basically the same image. This is essentially what TN and FStoppers did.

For years, the knock on Canon sensors was that their analog to digital conversion occurred off chip, thus there was significant noise added while transferring the analog signal off chip. Also, I've seen write ups that part of the "1D" treatment, was multiple analog-digital converters, thus faster, less heat generated and less noise. But, starting with the 80D and including the 5DIV/1DXII/M6/M5/M50/R (maybe others), my understanding is that these cameras have on chip AD conversion, thus less transfer and less noise. These cameras, when you look at their noise/DR results, are pretty linear. In contrast, the 6DII seems to still be off-chip conversion. Still a great camera, but this is why some on the internet bash the 6DII.

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Originally Posted by HDNitehawk

I have never heard that one, why would it?

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Yeah, I am pretty certain I picked up the "myth" somewhere. But, I picked it up, it made sense to me, and I saw data that fit the model (wavy noise results with varied ISO/quick change in noise results). But now, I can't find a single reference supporting this idea and have found several that seem to counter it.

So, I have areas of expertise, this is not one of them. It is always good to have some fluidity in your thoughts and opinions. Here, I am moving off this idea that different charges can be applied to a sensor and that contributes to ISO.

To explain the data, there has to be more to ISO than just gain added in one step to the analog signal. "Photons to photos" references dual gain to explain the wavy feature. Anyway, I'll post if I ever figure it out.

I actually did some testing with this invariance with my 5dIV.

What i did i took first picture of northern lights with settings like 4s iso 6400 f/1.8 then changed the settings to 4s iso400 and f/1.8. ISO 400 because apparently 400->5DIV is iso invarient. Then what i did on LR i boosted exposure that +4 and yes, the noise was close the same as taken with 6400. The dark areas on the landscape area do get some stripes what you get when boosting shadows too much etc. but the basics were pretty much the same.

I am wierdo when it comes to iso values when shooting northern lights that i don't use the so called base isos much i tend to use 2000/4000/5000 mostly when takin pictures. But what i need to test out still is that if taking the images like iso 800/1600 like i would do them with 4000/5000 and then do some +2-3 exposure upping, what kind of quoality i get there instead taking out some exposure from my bit over exposured ones.

Hey Kari,

It might not be totally analogous, but what you are describing is similar to discussions I've seen for astro work. Of course, astro work could be stacking images or single exposures, but there is a lot of good discussion.
http://dslr-astrophotography.com/iso-values-canon-cameras/
https://www.diyphotography.net/find-best-iso-astrophotography-dynamic-range-noise/


Both those links get into the whole ISO-variant vs ISO-Invariant discussion. The first links to a pretty good discussion of ISO that pretty emphatically states that ISO is just Gain/amplification to the analogue signal:
http://dslr-astrophotography.com/iso-dslr-astrophotography/

I am curious where you end up as we are both shooting 5DIVs. I do not often shoot nightscapes, but would love to shoot more, so anything you can pass along would be appreciated.

For what it is worth, and this does not seem updated, but a few years back, over at Clarkvision they settled on the 7DII for their low light work:

http://clarkvision.com/articles/characteristics-of-best-cameras-and-lenses-for-nightscape-astro-photography/

Basically, very little dark current noise at given temperatures.


Brant

I think maybe the steps you list for the sensor are for CCD, not CMOS sensors. Chuck says "I cannot speak for other manufacturers' products, but EOS Digital SLRs with Canon CMOS image sensors do not carry electrostatic charges at any time"

Seems like a lot of engineers on here...wonder if that says something about either engineers or photographers (or both)...

I read the same article Brant referenced from photographylife. That seems to make some sense to me, but I'm left with the question as to how some cameras and/or manufacturers can have such a low (practically zero) back end read noise while other cameras/manufacturers seem to have issues here. Surely it can't be that simple or beneficial or this would be something they would all be doing (or strongly trying to do).

Stephen

My wild quess on the read noise is what is the thing manufacturers want to favor. Apparently canon's idea is to have nice color space etc. which maybe then leads to issues in other areas. Not an engineer, but for me it looks like that the choice what is wanted from body is the somehow related to this.

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Originally Posted by DavidEccleston

I think maybe the steps you list for the sensor are for CCD, not CMOS sensors. Chuck says "I cannot speak for other manufacturers' products, but EOS Digital SLRs with Canon CMOS image sensors do not carry electrostatic charges at any time"

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Thanks. I just did a few searches to see if CCDs were handled differently from CMOS. It appears that the gain on a CCD is still to the analog or digital signals, same as a CMOS sensor. The more I look, the more references I find that there is just a myth out there that sensors have varied sensitivity. This is wrong. ISO should not be part of the "exposure triangle." Actually, there is no triangle. Rather, you have the light that hits the sensor controlled by aperture and shutter speed. Then gain can be added in camera (ISO) or in post processing (playing with exposure). In camera gain is usually added to the analog signal and post-processing is, of course, to the digital signal.

I believe chuck was talking about surface electrostatic charges that are rumored to attract particles. If he is right, this is another myth that is out there. I've always heard that sensors have an electrostatic surface charge. huh...well, this is what I like about being active on forums, I am still learning.

While I've seen this discussed several times, this video does a very nice job of laying out what a photodiode is, how it works, and the differences between CCD and CMOS:
https://www.youtube.com/watch?v=MytCfECfqWc

How the silicon sensor works, a diode, and the photoelectric effect are laid out from 2:50 to 5:30. A CMOS sensor is laid out at 9:04. Also, why CCDs are not great for modern cameras is laid out well in discussing the cons for CCDs starting at 8:10.

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Originally Posted by NFLD Stephen

Seems like a lot of engineers on here...wonder if that says something about either engineers or photographers (or both)...

I read the same article Brant referenced from photographylife. That seems to make some sense to me, but I'm left with the question as to how some cameras and/or manufacturers can have such a low (practically zero) back end read noise while other cameras/manufacturers seem to have issues here. Surely it can't be that simple or beneficial or this would be something they would all be doing (or strongly trying to do).

Stephen

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I've always been impressed by that correlation....lots of engineers (and some scientists) are hobbyist photographers.

For years the knock on Canon was too much background read/dark current/etc noise. This was largely attributed to having off sensor analog to digital conversion (ADC). Thus, the voltage for each pixel for each image had to migrate from the chip some distance before being converted to a digital signal. The longer the distance transferred, the more potential for slight changes to the voltage, which is noise. This affected blacks/shadows more simply because their voltage is so much lower than higher luminance.

So, if you have a signal of say 7 mV, and +/- 2 mV is added to it as noise during the transfer, that becomes an analog signal of 5 to 9. If you scale up after the ADC, say 3 stops (so 8x), that 5 to 9 luminance becomes 40 to 72. Had that same signal been set at ISO 800, so gain added before the off-chip transfer, then that 7 x 8 = 56 mV, then the +/- 2 mV noise is added and you end up with 54-58 mV range for the analog signal as it hits the ADC.

Sony gained a lot of notoriety for shortening this transfer distance and putting the ADC on the chip as the sensor. Canon has done that with some cameras (5DIV, 1DXII, 80D, etc) but only recently, and they still do not do that with all cameras.

The other primary difference that I've heard thrown around (but do not understand in its entirety) is chip architecture. Canon is somewhat unique in that they make their own camera sensors. Nikon buys theirs. Sony also makes chips and really got into making chips for camera phones.

This article is from 2012, so it is dated, but at the time Canon was still using a chip fabrication based on 0.5 microns (my understanding is this is the substructure that the chip is built upon). Sony, even in 2012, had 0.18 micron and 0.25 micron substructures. Again, Civil Engineer here, but smaller process generation allows for the construction of smaller, more efficient (less noise) systems on the chip level. It also seems that the smaller substructure may be important for moving the ADC onto the chip.

My understanding is that, again, 5DIV, 80D, 1DX II, chips are all built using a smaller process generation. I want to say I remember 0.25 micron, but I can't find that link to confirm, so maybe it is 0.18 micron that was referenced in the article. But, the fact that Canon has an existing "mature" 0.5 micron process generation system, they still occasionally pump out sensors with "old" technologies (hello 6DII) that it is inferred are based on the mature 0.5 micron fabrication (I've never seen this proven, but generally assumed).

So, there are a lot of differences between manufacturers. Patents likely ensure that will continue. Personally, while many focus on the above, Canon does many things extremely well and, as a photographer, I really have no issues with the images coming off my 5DIV and I had very few issues with my 5DIII and its "older" sensor fabrication. In fact, many of my all time favorite images were taken with the 5DIII. While I love the technical details, photography is still about capturing moments, scenes, and light.

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Originally Posted by Kayaker72

here, but smaller process generation allows for the construction of smaller, more efficient (less noise) systems on the chip level. It also seems that the smaller substructure may be important for moving the ADC onto the chip.

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Larger pixel = Less Noise due to the S/N ratio.
Smaller more efficient structure would allow for better handling but you end up with an unfavorable S/N ratio to deal with as pixels get smaller.

Since you posted this I have been doing some reading, I think to develop a good understanding a line diagram would be helpful. There are multiple steps to remove various types of noise during the process.

Quote:

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Originally Posted by Kayaker72

This is wrong. ISO should not be part of the "exposure triangle." Actually, there is no triangle. Rather, you have the light that hits the sensor controlled by aperture and shutter speed.

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I do not agree with this statement. You have to have ISO because in many cases the required shutters speed and aperture required is outside of what is possible. You have to have the third element to compensate.

ISO is the boost in gain of the signal for a digital camera. Much of what you are talking about is how it is handled in camera and in computer.


I did a quick google search. This miss-leading explanation popped up, this is a dumbed down statement for the masses. Most people this explanation will suffice even if it is not accurate. You really do not need a higher understanding to take great pictures.

"In Digital Photography ISO measures the sensitivity of the image sensor. The same principles apply as in film photography – the lower the number the less sensitive your camera is to light and the finer the grain. ... By choosing a higher ISO you can use a faster shutter speed to freeze the movement."

If any part isn't real it is "ISO Invariance", it is nothing more than your ability to adjust the gain at a different point other than inside the body of the camera. Why would you even want to do this and underexpose pictures, so you can chimp your black pictures? What would really be the functional use for this "knowledge", other than you now know you can adjust your ISO in post without penalty?

Quote:

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Originally Posted by HDNitehawk

Larger pixel = Less Noise due to the S/N ratio.
Smaller more efficient structure would allow for better handling but you end up with an unfavorable S/N ratio to deal with as pixels get smaller.

Since you posted this I have been doing some reading, I think to develop a good understanding a line diagram would be helpful. There are multiple steps to remove various types of noise during the process.




I do not agree with this statement. You have to have ISO because in many cases the required shutters speed and aperture required is outside of what is possible. You have to have the third element to compensate.

ISO is the boost in gain of the signal for a digital camera. Much of what you are talking about is how it is handled in camera and in computer.


I did a quick google search. This miss-leading explanation popped up, this is a dumbed down statement for the masses. Most people this explanation will suffice even if it is not accurate. You really do not need a higher understanding to take great pictures.

"In Digital Photography ISO measures the sensitivity of the image sensor. The same principles apply as in film photography – the lower the number the less sensitive your camera is to light and the finer the grain. ... By choosing a higher ISO you can use a faster shutter speed to freeze the movement."

If any part isn't real it is "ISO Invariance", it is nothing more than your ability to adjust the gain at a different point other than inside the body of the camera. Why would you even want to do this and underexpose pictures, so you can chimp your black pictures? What would really be the functional use for this "knowledge", other than you now know you can adjust your ISO in post without penalty?

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Hi Rick...No, this is not an attempt to say never use ISO. In terms of practice, I have every intention to continue to use ISO as it makes little sense to run around shooting black images and add gain in post. But, in some instances, I might use ISO differently and hopefully, better.

And how to best articulate this is still evolving, but I think we can agree that ISO is something that occurs after the image capture (electrons captured-initial analog signal generated) . Yes, in actual photography when you want to control exposure, ISO will influence your selection of shutterspeed and aperture. So, ISO's influence on exposure is indirect as in it does not directly influence the sensor itself.

It is, of course, a good question of how to use this type of knowledge. First, ISO is not a lossless process. Adding ISO, or gain, at the analog stage causes you to lose information as ISO raise the floor, but the ceiling does not change. So, if your goal is to maximize dynamic range and do care about shutterspeed or aperture, you should shoot at the lowest ISO where your camera becomes ISO invariant. Then, in post, you can control whites, highlights, midtones, shadows, and blacks separately. In short, shooting at the lowest ISO where your camera becomes ISO-invariant maximizes the information you have to work with in post. I've had a rudimentary appreciation for this, but working through this thread is highlighting this approach (this is what astro photographers are doing).

But really, ISO invariance is not a cause, it is a result of a lower noise floor, better handling of the ADC, and overall cleaner post image capture process. There are lots of uses of that knowledge and understanding (granted, each is camera specific). In my rudimentary understanding, I was already shooting a bit differently for high contrast scenes. For example, I will shoot a loon with more light on it than before, knowing I can expose for the white breast and bring up shadows in post. I have seen references to landscape photographers shooting with few exposures in an exposure bracket to capture a scene. Then, as mentioned, I am seeing astro- or nightscape photographers use ISO invariance to optimize their settings. I am still digging into this, but part of this has made me recall some videographer discussions I saw a year plus ago that talked about controlling dynamic range above and below "proper exposure" (18% gray) and maximizing dynamic range at different ISOs. As video is baked in jpeg equivalents, this makes sense.

So, I am still working through some of this, but thinking of ISO as gain that is applied to the analog signal rather than something that inherently and directly impacting the image on the sensor can change a few things. In the future, I will be thinking more about when it is best to add gain, in camera (often) or in post (sometimes).

I mentioned a line diagram would be useful. From what I have read it might not be an equal trade. Noise is removed at several points during the process. Would out of body be equal to doing it the way the camera is designed, or is there a penalty?

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Originally Posted by HDNitehawk

I mentioned a line diagram would be useful. From what I have read it might not be an equal trade. Noise is removed at several points during the process. Would out of body be equal to doing it the way the camera is designed, or is there a penalty?

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Agree that a line diagram would be helpful, but I don't think there will be one forthcoming. Each camera could have a slightly different process and certainly different amounts of noise added at various stages.

And as I understand it, true iso invariance would have no penalty at all. But I suspect that in practice very few cameras are truly iso invariant and that there will always be some difference between noise in an image shot at high iso and an image brought up in post.

As for advantages, one of the primary one's I've heard is regarding insurance against not blowing the highlights. Many people practice ETTR to maximize data gathered, but this risks having something overexposed and clipping the highlights (which can't be recovered). By exposing a bit lower in-camera your much less likely to clip the highlights and if the camera is iso invariant then you're not even paying a penalty to bring up the shadows in post.

A true electrical engineering line diagram would be another level, but if it helps, here are two cartoonish line diagrams I lifted from the net:

Attachment 2772



Attachment 2773


Then this is talking about noise, but also includes some description of the process:
Attachment 2774

and, while I am at this (as this actually started for me looking at sensor size, and quantum efficiency, but those posts are coming :D)
Attachment 2775


So, a quick summary of my understanding of the process (and I know this thread started with me being on the fence, but I quickly tipped over to this line of thinking):

1. Photons hit a photodiode (charged) usually referred to as a sensor. Photons bump electrons off the charged photodiode. The ratio of electrons measured to the photons that hit the sensor is the quantum efficiency. But # photons hitting the sensor is about the light which is about shutterspeed and aperture. ISO only comes into play as it affects the selected shutterspeed and aperture.
2. Electrons are captured in the pixel well.
3. For a CMOS sensor, the captured electrons are converted to a voltage at the pixel level
4. ISO is gain that is added to the analog signal
5. The amplified or native analog signal is transmitted to an analog-digital converter (ADC or I sometimes see ADU)
6. The created digital signal is still just for luminance, it is then processed into a RAW file taking into consideration a number of factors including the Bayer Filter, white balance, etc.
7. The digital signal can be adjusted (up or down) at the computer by the photographer. There may also be gain adjustment by the camera, as I mentioned, I am still trying to understand dual gain by Sony and the wavy curves generated by Canon sensors.

So, in a way, I also want to minimize the discussion of "ISO Invariance" as really that is simply the realization that this process has been optimized to the point that there is minimal (no?) penalty for adding gain at Step 4 vs Step 7. It does not mean that you want to suddenly change and start shooting everything at native ISO or at the point your camera becomes invariant and add gain in post. But, in some instances, you have that option more than you did a few years ago.

While I started this thread mostly because I saw something go by that was different than my understanding and I have come around to ISO being something that is added downstream of the sensor, if you are interested in the whole "ISO is Fake" discussion, I just read an article from Thom Hogan where I think he makes some good points:
http://dslrbodies.com/cameras/camera-articles/image-sensors/is-iso-fake.html

Quote:

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Originally Posted by HDNitehawk

I mentioned a line diagram would be useful. From what I have read it might not be an equal trade. Noise is removed at several points during the process. Would out of body be equal to doing it the way the camera is designed, or is there a penalty?

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I have yet to come across noise being removed, unless you mean that it is minimized in modern cameras compared to before or minimized with something like faster shutterspeeds. But for the same camera, I see noise as being inherent to different processes. Now are there times when adding upstream noise by adjusting ISO better than adding noise downstream. Yes. Absolutely. That is much of the point of what astro photographers are doing and summarized in this article:

https://www.diyphotography.net/find-best-iso-astrophotography-dynamic-range-noise/

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Originally Posted by NFLD Stephen

Agree that a line diagram would be helpful, but I don't think there will be one forthcoming. Each camera could have a slightly different process and certainly different amounts of noise added at various stages.

And as I understand it, true iso invariance would have no penalty at all. But I suspect that in practice very few cameras are truly iso invariant and that there will always be some difference between noise in an image shot at high iso and an image brought up in post.

As for advantages, one of the primary one's I've heard is regarding insurance against not blowing the highlights. Many people practice ETTR to maximize data gathered, but this risks having something overexposed and clipping the highlights (which can't be recovered). By exposing a bit lower in-camera your much less likely to clip the highlights and if the camera is iso invariant then you're not even paying a penalty to bring up the shadows in post.

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Agreed...there is some back end noise added. I doubt true invariance exists. But, some of those DR/SNR/Noise lines are pretty darn straight, so some cameras are getting very close.

One comment I made earlier in the thread was about ETTR and how it is less important. I think you just said it better. You can increasing the distance you put between blowing out highlights and your exposure. I've also been looking at articles on eye sensitivity/perception and people are more sensitive to shadows than highlights. So, while digital sensors gain more information from brighter images, people perceive more detail in the shadows. ETTR is basically trying to merge those two facts by gathering as much data where digital sensors are good at gathering it, but then dropping exposure in post and using that data where people perceive it better. But, modern sensors are now getting better in the shadows having lower noise floors and overall less shadow noise. So, ETTR is probably still useful, but perhaps not as critical as it was?

Where this is evolving for me is just trying to understand what is happening better so I can adjust, as needed, better. Also, I just like understanding things (:D).

Quote:

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Originally Posted by Kayaker72

I have yet to come across noise being removed, unless you mean that it is minimized in modern cameras compared to before or minimized with something like faster shutterspeeds. But for the same camera, I see noise as being inherent to different processes. Now are there times when adding upstream noise by adjusting ISO better than adding noise downstream. Yes. Absolutely. That is much of the point of what astro photographers are doing and summarized in this article:

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I came across a white paper from Canon that talked about the structure of the sensor. Every step in the process is designed with decreasing noise in mind. For the theory that there is no penalty the ISO Settings in camera must = the EV bar in LR.

This is a comment in your linked article:
"Upon comparison of the exposures, it’s immediately apparent that the Canon EOS 700D/T5i is not completely ISO-invariant"

This would lead me to believe that there are process in the camera that take place that are not completely replicated in LR.

Another article I was reading said that most Canon bodies are not all invariant.

This sounds like a rehash of the Sony DR argument / debate.

Start on page 17

http://www.robgalbraith.com/images/c...hite_paper.pdf

Quote:

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Originally Posted by HDNitehawk

I came across a white paper from Canon that talked about the structure of the sensor. Every step in the process is designed with decreasing noise in mind. For the theory that there is no penalty the ISO Settings in camera must = the EV bar in LR.

This is a comment in your linked article:
"Upon comparison of the exposures, it’s immediately apparent that the Canon EOS 700D/T5i is not completely ISO-invariant"

This would lead me to believe that there are process in the camera that take place that are not completely replicated in LR.

Another article I was reading said that most Canon bodies are not all invariant.

This sounds like a rehash of the Sony DR argument / debate.

---

Thanks for the article, that was a great level of detail at the pixel level.

As for the Sony DR rehash...maybe. I never had an issue when people were talking about how good Sony sensors are/were. The numbers are there and there is a reason why so many other companies (Nikon, Apple, etc, even Canon for 1" sensors) buy Sony sensors. The issue I often had with the Sony DR debate was the sentiment that Canon sensors were somehow awful and that everything else that Canon did well was insignificant. Also, there was a tendency to give Sony a free pass on everything else (AF, lens lineup, weather sealing, battery life, etc) just because they had sensors that were marginally better. To me, it was a discussion about needles while missing the forest type of debate.

Now, Canon has mostly caught up with Sony with sensors but only certain cameras like the 5DIV, 1DX II, etc (not the T5i).

But, if you want to characterize "iso-invariance" as a discussion regarding the evolution of sensors, by Sony and Canon, and the characteristics of those new sensors, then, yeah, there is some overlap. Lower noise, more DR, etc. One of the things that comes from that is a degree of iso invariance. Whether it is useful or not to your photography is really up to you. But, it is a characteristic of certain cameras over a wide range of ISOs or of most cameras in a selected range.

If you want, you can check out dpreview's comparison tool designed to do just that:
https://www.dpreview.com/reviews/canon-eos-5d-mark-iv/11

If you want a real world application. Here is the baseline of a shot I posted about a year ago where I exposed for the sky to try to get the sun as a fairly tight circle right before sunset:

Attachment 2776

This includes a 3 stop grad ND filter on the sky trying to get the overall exposure up.

But, in post I bumped up the exposure by 3.1 EV and 62 of shadows which seems to have lifted the shadows by ~4.5 stops overall. This is the picture I posted:

https://live.staticflickr.com/789/26...e8526ba2_b.jpg6S0A7685-3 by kayaker72, on Flickr

I do not do this often (and actually had I not been pressed for time here, I would have tried to bracket this shot), but it does have its uses.

But, overall, I think iso invariance is what explains why there are videos by Tony Northup/Fstoppers out there shocked that you can underexpose an image and lift it in post by ~5 stops and have it be about equal to something shot with 5 stop higher ISO. I was actually about to reply, but realized my concept of ISO was perhaps off. Through this thread (comments and research) I've confirmed that ISO is in camera analog gain (maybe some in camera digital gain). The iso-invariance comes into play as camera manufacturers are doing a better job controlling "downstream" noise. As that is better controlled, digital gain in post is getting closer to analog gain applied in camera. Of course, this is going to vary from camera to camera.

Maybe Sony is doing less control than Canon in body and that is why Sony is more "ISO Invariant".

From the link in your last post:

"How does the 5D Mark IV stack up against some of its other peers? Its performance is much better than any other offering from Canon, but it's still not quite as ISO-invariant as the Nikon D810 or D750,"

Perhaps Sony has given the illusion that its ability to handle DR is greater than it actually is.


This quote, it really is another version of the DR discussion just said in a different way with different terms.

The 5D Mark IV isn't entirely ISO-invariant: pushing an ISO 200 underexposed by 5 stops by 5 EV in post-processing yields slightly higher noise levels than a native ISO 6400 exposure. An ISO 100 exposure pushed 6 stops fares even worse. However, above ISO 400, the camera does, for the most part, exhibit ISO invariance, meaning that you could underexpose a traditional ISO 6400 exposure by 4 EV by shooting it at ISO 400 (while maintaining the shutter speed and aperture for ISO 6400), and then raise exposure 4 EV in post. This technique would afford you 4 EV of highlight headroom, with little to no noise cost, relative to shooting at ISO 6400.

I took 2 pictures last night one with 4s iso 6400 f/1.8 and other same but iso 800 will post later on so that i sync wb and tint for both images and then boost exposure +3 on the iso 800 so we can have some comparision that lvl. body is 5dIV

First off, understanding all this does help. Right now, I think the

astrophotographers

have it figured out, likely because it helps that type of photography the most. At least three astrophotography blogs I found focused on where a camera becomes ISO invariant. They have focused on the ISO where upstream noise (generated by increasing ISO) is greater than downstream noise. Looking at charts, I would agree with you/dpreview concluded, for the 5DIV, it is about ISO400.

But, dpreview bases all their "ISO invariance" tests on ISO 100, ignoring the fact that different cameras become invariant at different ISOs. Maybe they get it and are only concerned about base ISO. Then, some of the tests TN/Fstoppers did could be explained by a dual gain sensor (TN figured it out in the second video).

So, sensors have different characteristics, from megapixels, to pixel size, etc. I think this is exploring a lesser known characteristic.

Quote:

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Originally Posted by HDNitehawk

Maybe Sony is doing less control than Canon in body and that is why Sony is more "ISO Invariant".

From the link in your last post:

"How does the 5D Mark IV stack up against some of its other peers? Its performance is much better than any other offering from Canon, but it's still not quite as ISO-invariant as the Nikon D810 or D750,"

Perhaps Sony has given the illusion that its ability to handle DR is greater than it actually is.


This quote, it really is another version of the DR discussion just said in a different way with different terms.

The 5D Mark IV isn't entirely ISO-invariant: pushing an ISO 200 underexposed by 5 stops by 5 EV in post-processing yields slightly higher noise levels than a native ISO 6400 exposure. An ISO 100 exposure pushed 6 stops fares even worse. However, above ISO 400, the camera does, for the most part, exhibit ISO invariance, meaning that you could underexpose a traditional ISO 6400 exposure by 4 EV by shooting it at ISO 400 (while maintaining the shutter speed and aperture for ISO 6400), and then raise exposure 4 EV in post. This technique would afford you 4 EV of highlight headroom, with little to no noise cost, relative to shooting at ISO 6400.

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I've wondered about Sony as well. The whole "star eater" issue they had, it seemed that their RAW algorithm was looking for noise and crushing it to blacks and happened to eat a few stars in the process. Of course, this is an artificial/destructive way to create black/shadows with less noise. From what I've seen, the current algorithm is better, but I do wonder if it is still going on, just more refined. Then, I am still looking at this "dual gain" system of theirs. If ISO is post pixel, it almost has to be a digital manipulation.

I've also wondered about Canon vs Sony's Bayer filters. How many photons are lost at the Bayer filter? I do not even see this discussed (other than by those praising Foveon sensors that do not have a Bayer filter). But, thinking about it, theoretically if you defined "Red," "Green" and "Blue" as very narrow, your colors would be more accurate, but you would have to block out more light. Define Red/Green/Blue more broadly, more photons make it past the Bayer filter, but your colors are likely less accurate. So, in DXOmark, if you just look at the measurements for an individual camera sensor you can see their estimated accuracy in color reproduction (ISO 17321). For the cameras I've looked at, Sony and Canon are pretty equal, and no one talks about this, making me think camera manufacturers handle this similarly, likely with widely defined colors, but still something I've wondered about considering Canon is generally praised for colors.

All that said, I do think Sony decreased downstream noise with on chip ADCs decreasing the travel distance for the analog signal. I've also read Sony has more ADCs than Canon typically has, which would decrease heat, noise, etc. Then, in looking through Thom Hogan's articles yesterday, I found this. If this ever comes to market, this next generation of sensor will read "0" as pure white and "max" signal as pure black. What this does is shifts low end noise to the whites, where it will hardly be noticeable and blacks will have almost imperceptible noise. This is cool. Technically, it is either an odd manipulate of the pixel wells, or they are doing something like reading pixels lost by the photodiode and not pixels gained and held in the pixel well. Something, but still, I have to give credit where it is do, this, at a minimum, is a cool concept.

Quote:

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Originally Posted by Karsaa

I took 2 pictures last night one with 4s iso 6400 f/1.8 and other same but iso 800 will post later on so that i sync wb and tint for both images and then boost exposure +3 on the iso 800 so we can have some comparision that lvl. body is 5dIV

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Great, I've been thinking of doing some tests, so it will be great to see yours.

Ultimately, this was about understanding the system for me. It is interesting that several astrophotography blogs were definitely on top of this subject, but I do wonder how much of a difference it will make.

Quote:

---

Originally Posted by Kayaker72

First off, understanding all this does help. Right now, I think the

astrophotographers

have it figured out, likely because it helps that type of photography the most. At least three astrophotography blogs I found focused on where a camera becomes ISO invariant. They have focused on the ISO where upstream noise (generated by increasing ISO) is greater than downstream noise. Looking at charts, I would agree with you/dpreview concluded, for the 5DIV, it is about ISO400.

But, dpreview bases all their "ISO invariance" tests on ISO 100, ignoring the fact that different cameras become invariant at different ISOs. Maybe they get it and are only concerned about base ISO. Then, some of the tests TN/Fstoppers did could be explained by a dual gain sensor (TN figured it out in the second video).

---

Sounds familiar to the old DR discussions. Sony is obviously better.

Most of the discussion is centered around camera bodies. Really that may only be half of the equation in the discussion. Don't you think it would be just as important to understand what is happening with your PP software? Or are we assuming all PP programs are equal and as such invariant? Do all the PP programs simulate the gain boost just like the camera?

Here as promised:

6400 4s f/1.8

https://live.staticflickr.com/7919/3...b8b15400_k.jpg6400 by Kari, on Flickr

800 4s f/1.8 +3 exposure

https://live.staticflickr.com/7882/4...637e81fd_k.jpg800 by Kari, on Flickr

Quote:

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Originally Posted by HDNitehawk

[/LEFT]

Sounds familiar to the old DR discussions. Sony is obviously better.

Most of the discussion is centered around camera bodies. Really that may only be half of the equation in the discussion. Don't you think it would be just as important to understand what is happening with your PP software? Or are we assuming all PP programs are equal and as such invariant? Do all the PP programs simulate the gain boost just like the camera?

---

Ha :) that sounds like another rabbit hole that I could spend awhile exploring. Which software is better. I will say, I did come across the following by Thom Hogan while reviewing the sensor performance:

Here, he talks about the ISO invariance and brings up the software as well. Fairly interesting, his point (and the point of other program developers) is that programs like adobe operate as 16 bit even though our computers are typically 64 bit. This is to cut down on computational time and enhance the speed of processing. Granted, our files are 14 bit, but the articles point is that information is lost and working in higher bits is better for overall accuracy. They talk about RAWDigger and "Raw Photo Processer" being more accurate. Granted, he consulted at least one of the software engineers behind those processers for the article.

Then Here, Thom states his opinion as to the best software varies by manufacturer, adobe caters to the largest (i.e. Canon), while Capture One has some deal going with Sony, etc.

Quote:

---

Originally Posted by Karsaa

Here as promised:

---

Thanks....the biggest difference is the purple streak going across the top of the scene in the ISO 800 shot +3stops. Contrail from a plane that went by?

Otherwise, I still give it slightly to the native ISO 6400 shot. A bit more pattern noise while adding 3 stops to the ISO 800 image. I did have to look on flickr and zooming in helped, so close, but I'd give it to native ISO.

Great colors by the way, the subtle transitions are excellent (oops, I know, I just talked about actual photography in a technical discussion...my bad....;))

Yeah, i can say that i prefer also shoot with iso i need not twidle with sliders =) I would say +3 gives more color noise. That is what comes when you boots exposure...or if not then my shutters is getting angry from last little time lapses. That line i mean.

Yeah, my first impression was more chroma noise too. I wondering, when you apply +3 exposure, and noise reduction, does ACR do the noise reduction before or after scaling everything * 8.

What if you convert to 16-bit w/ just noise reduction, then once in Photoshop, boost the exposure. This will force the order of operations, and perhaps remove the increase in chroma noise.

So, just ran some quick tests of my own. Now that I am inside and looking at the pictures, I am pretty sure my neighbor's motion lights came on for a couple of the images, but not others.

That said....for the first time with my 5DIV, I am seeing banding. I had actually looked before. Not a big deal. Still love the camera and I really do not want to devolve to obsessing about pics raised 4-5 stops. While I was hoping there was some reason for the purple line in Kari's shot, it is likely banding as well.

But, as I have been doing searches the last couple of days on this topic, I thought I'd do a quick one on banding. I found this, written by jrista, for those that were on CR a few years ago, you may recall, intelligent, detailed, DR focused posts that not everyone agreed with. Great astro images as well.

As this thread is about understanding the process, if jrista is correct, it actually goes a long way to explaining things. According to jrista's response to a question, what we've been describing above, where ISO is gain added to the analog signal, well, that is Canon (and maybe other manufacturers). It is not the process for Sony.

According to jrista's post, Sony did away with analog gain (ISO) with the exmor sensor. Not only did they increase the number of ADCs (one per column, which I've read before), they took the sensor output and directly digitized it before adding gain.

So, of course Sony is "iso-invariant," ISO in exmor land is in camera digital gain vs post processing digital gain.

But, they are also doing another "trick." And this gets back to the article Rick posted which I'll have to go back to read as I did not fully understand this part. But after a pixel is read, then there is a reset charge. Well, Sony is reading that reset charge and subtracting it from the next images signal. Thus, random noise generated while reading the reset charge (plus the reset charge) is subtracting out noise from the actual image (plus a little extra). So, it is possible for a Sony (and Nikon using Sony exmor sensors) to have a negative number, but lets get real, they probably define that as 00000000000000.

From the posts so far is it a conclusion that raising the shadows of the 5D IV 4-5 stops is not acceptable, but 3 might be.
And
Sony and Nikon are better for astrophotographers.
And
Daniel Browning no longer posts on this forum.

We might go back and look at some of Daniel's complaints about Canon. But if my memory is correct this kind of summed up why we no longer see him posting.

Ha! Very true about Daniel. Had some great, but very technical, posts (and also great pictures). But I learned a lot from him. His participation dropped off after he switched to Nikon because he was tired of the issues with Canon which weren't fixed with the 5DIII/1DX releases in 2012. But, one of the last posts of his that I remember he came back with a long list of issues he had with Nikon and their sensor tech (Sony exmor sensor).

Grass is just different shades of green depending on where you are. :)

Maybe different shades of green depending on your sensor:rolleyes:

Little off topic but been shooting now milky way and northern lights with my friend who has also the sony a7III. When he's not shooting timelapse etc with sony he is having the liveview open so its very easy to see when auroras start to appear, and now it has software update for having intervalometer with some fancy settings included i am starting to fade toward sony as my 2nd body instead of R. Sony has one issue that most likely comes from the being canonist whole time that its colors when the sky has some colors on it tend to tint..was it towards purple and fixing that is pretty much annoying. Unless canon deside to give soft update to R with build in intervalometer....looks sony for me :P

Odd. The 7DmkII has an intervalometer built-in. You would assume any camera after that would also include it, except perhaps the Rebel series (gotta force those upgrades somehow, no matter how ridiculous.)

Jrista has some heavy cred in the astro stuff on c.rumors, i suspect he gets sensors way more than most.

I miss Dans comments always helpful, wish he would hang out here.

https://jonrista.com/2015/03/01/dynamic-range/