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Thread: ISO Invariance

  1. #11
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    Quote Originally Posted by Kayaker72 View Post
    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.


    Quote Originally Posted by Kayaker72 View Post
    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?

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    Quote Originally Posted by HDNitehawk View Post
    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).
    Last edited by Kayaker72; 04-09-2019 at 08:03 PM.

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    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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    Quote Originally Posted by HDNitehawk View Post
    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?
    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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    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:

    Name:  Line - image capture.jpg
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    Name:  Line - image capture 2.jpg
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    Then this is talking about noise, but also includes some description of the process:
    Name:  Noise Illustrated .jpg
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    and, while I am at this (as this actually started for me looking at sensor size, and quantum efficiency, but those posts are coming )
    Name:  Quantum Eff pixel.jpg
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    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 Originally Posted by HDNitehawk View Post
    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?
    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/

    Quote Originally Posted by NFLD Stephen View Post
    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.
    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 ().

  6. #16
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    Quote Originally Posted by Kayaker72 View Post

    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:
    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, its 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.

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  8. #18
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    Quote Originally Posted by HDNitehawk View Post
    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:

    Name:  TDP-7685.jpg
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    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:

    6S0A7685-3 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.


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    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 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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