ISO Invariance

[Kayaker72]

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"

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.

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

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.

[HDNitehawk]

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.

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.

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.

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?

[Kayaker72]

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?

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).