Thread: ISO - Useless while shooting and in post

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  1. #1 ISO - Useless while shooting and in post 
    Senior Member Rui Guerra's Avatar
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    Hi all,

    Thinking about what ISO change does or not, it looks like a pretty much useless control (if shooting RAW).
    Do you agree ?

    Well, let's see:

    WHILE SHOOTING:

    1 - We agree that choosing an ISO value in camera while shooting is the same as choosing it in post, if everything else stays the same, since ISO is just metadata that can always be changed afterwards.
    2 - We also know that lowering the ISO leads to more stops of dynamic range in the shadows (less noise and more shadow detail) and higher ISO values will keep more stops of dynamic range in the highlights (protection against burned highlights). Dynamic Range is always the same, it's just displaced towards one end or the other.
    3 - But the statement in number 2 is always valid, no matter if a particular ISO number is chosen while shooting or in post.
    4 - Although that many people follow the thinking that choosing a low ISO while shooting in low light will lower the image noise, that is only true if besides the ISO lowering, something else also change (lighting, aperture or shutter speed).

    Conclusion A - If everything above is correct, changing only the ISO (while shooting), doesn't do anything to the image. If so, any change in image quality are only made when one physically change the amount of light that reaches the sensor (by changes in aperture, shutter angle, lighting or ND filters).

    IN POST:

    5 - Again, what is said in number 1.
    6 - We know that to make a good color correction and grade in post, for example in DaVinci Resolve, we should start with a flat log image (IPP2/RedWideGamma/Log3D10).
    7 - If we look inside the camera RAW settings inside the Color tab of DVResolve, we can see that ISO is set to 320, regardless of the type of shooting conditions (bright or dark) or the ISO value that we've chosen while shooting.
    8 - While we advance in the color correction and grading of our image, we never go back to those camera RAW settings and adjust ISO. We just keep using the primaries and secondaries adjustments, taking advantage of the very wide dynamic range of that initial flat/log image, that contais all the light and color information that a RED sensor can record.

    Conclusion B - Is everything above is also correct, changing the ISO in post is useless, as is also useless any time spend to choose a specific value while shooting.
    Conclusion C - The displacement of the Dynamic Range according to the ISO value (explained in number 2) is also completely irrelevant in post, since we just use as a starting point the flat/log image.

    GENERAL CONCLUSIONS:

    D - We can further conclude that the ISO control is completely useless, both in camera and in post. So all we need is the camera's tools to check the amount of light reaching the sensor (goal posts, traffic lights, GIO Scopes, etc) while shooting, and a flat/log image while editing. We can disregard ISO value completely in both stages.

    E - The displacement of the Dynamic Range towards the shadows or towards the highlights (also frequently referred to as the level of mid grey), or in fact the knowledge about that fact, is also irrelevant because 1) we don't need to set ISO at any specific value while shooting and 2) we we'll not use it in post anyway.

    F - ISO can be used just as a reminder of some light controlling decisions that should be made while shooting, but if one knows how to use the camera's exposure tools, there is no need to change ISO for that.

    So, the final big question: do you agree that ISO is an irrelevant control that we can forget about, or there is any specific scenario (while shooting or even in post) where you think that ISO plays a main role?

    All the best,
    Rui Guerra - PHOTOGUERRA Underwater Productions, Lda.
    www.photoguerra.net
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  2. #2  
    It works like this. Iso is digital gain added as a parameter post capture.

    So for example the DP shoots a couple of wide shots at a certain stop. Then when he moves in to shoot his closeups he might want less focus falloff, so he close down two stops. Then to make close look the same as his wide in therms of brightness he can upp the iso for the closup by multitude of 2. If the wide was 800 he would simply change iso to 3200 for the closeup.

    Now if the colorist does things right he import the footy with camera setting so in his timeline the wide and the tight mach up. The got the same brightness values, one is less exposed and has higher iso and the other vice versa.


    But yes when you shot iso irrelevant, what matters is how you expose the sensor and iso does not do thing to your exposure, at that point is like brightness of the screen. But the iso and color temp etc that you set tells the colorist / dit or who ever is next to work on the material what your intentions where.
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  3. #3  
    Senior Member Rui Guerra's Avatar
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    Hum... if the wide shoot is well exposed, following that procedure will make the close-ups 2 stops underexposed.

    If the DP needs to gain depth of field for the close-ups and wants to maintain a "brightness" level for all shoots, he should than correct the light/exposure for the close-ups (more natural/artificial light or lowering shutter speed), avoiding that way a noisier underexposed close-up images.

    Using ISO that way will be like using it as a "post-it note", but a dangerous one, regarding image quality. Don't see any advantage in that.
    Rui Guerra - PHOTOGUERRA Underwater Productions, Lda.
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  4. #4  
    Senior Member Audy Erel's Avatar
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    Constant ISO number only needed to keep the noise/grain consistency across the entire film.. Other than that it only act like a 'monitor brightness setting'.
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  5. #5  
    Senior Member Rui Guerra's Avatar
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    Quote Originally Posted by Audy Erel View Post
    Other than that it only act like a 'monitor brightness setting'.
    Indeed, that's a good description.

    But then, if in camera ISO is just a "post-it", a reminder (so it exacerbates the need to take measures regarding shadows or highlights) or just a "monitor brightness setting", then why there are so many talk about the advantage of RED ISO been a range with constant DR, and the "important" practice of lowering it in low light situations to gain more stops below middle grey (and the opposite for bright conditions).

    In the end, it looks like all that information is pointless since changing it in camera doesn't do anything per si and in post what we need is just a flat/log image.
    Rui Guerra - PHOTOGUERRA Underwater Productions, Lda.
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  6. #6  
    Senior Member Audy Erel's Avatar
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    I think the 'low ISO for clean shadow' principle only works if we have full control of all the lighting.. Don't confuse "low key" (a creative decision) with "low light" (lack of light) though, totally different beasts. We need MORE light to produce deep & clean low key images (hence, the darker monitoring or lower ISO).

    The 'higher ISO for highlight protection' principle on the other hand, is more applicable to wild documentary/available lights only situation where we could not help with the blown out windows or bright skies but to reduce the light hitting the sensor (with ND, Aperture or Shutter Speed) thus making all seems darker as a consequence to have that lovely clouds correctly exposed, so in turn we need to "brighten the monitor up" to make everything else look right, (a.k.a. raise the ISO)!

    Given no creative look nor grain consistency is concerned, the best practice with RED or any raw camera is to expose to the right. Go as far right as possible with the GIO scope without overexposing highlight and you will be good. We could always add grain later in post but we could never get the lost highlights back. It's all safely contained in the flat log image.

    But remember the usable DR is actually quite limited... It's a constant battle between the noise floor and highlight clipping. We need to choose where to shift the middle grey according to our need/priority.
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  7. #7  
    Senior Member Rui Guerra's Avatar
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    Quote Originally Posted by Audy Erel View Post
    I think the 'low ISO for clean shadow' principle only works if we have full control of all the lighting.. Don't confuse "low key" (a creative decision) with "low light" (lack of light) though, totally different beasts. We need MORE light to produce deep & clean low key images (hence, the darker monitoring or lower ISO).
    Yes but then changing the ISO is just a commodity (to improve monitor visualization).Darkening the monitoring can of course also be done by lowering the monitor brightness (to a lesser degree, depending on the monitor). In either case, we agree that in reality lowering ISO doesn't do anything to the image, and ultimately is not needed.

    Quote Originally Posted by Audy Erel View Post
    The 'higher ISO for highlight protection' principle on the other hand, is more applicable to wild documentary/available lights only situation where we could not help with the blown out windows or bright skies but to reduce the light hitting the sensor (with ND, Aperture or Shutter Speed) thus making all seems darker as a consequence to have that lovely clouds correctly exposed, so in turn we need to "brighten the monitor up" to make everything else look right, (a.k.a. raise the ISO)!
    Again, the raison is just to make some things right, not a technical reason indeed. Just by looking at the traffic lights we know when something will be burned out and take measures to prevent it, as you said. No need to change ISO. BTW, rising the ISO so "everything else look right" will again display the sky almost white. So this is just a personal choice of what we want to see better in the monitor. So, no technical reason here also.


    Quote Originally Posted by Audy Erel View Post
    Given no creative look nor grain consistency is concerned, the best practice with RED or any raw camera is to expose to the right. Go as far right as possible with the GIO scope without overexposing highlight and you will be good. We could always add grain later in post but we could never get the lost highlights back. It's all safely contained in the flat log image.
    I could not agree more! And that's the main focus of this post. Besides the usual exposure correct practices (ETTR), the bottom line is "It's all safely contained in the flat log image."

    Quote Originally Posted by Audy Erel View Post
    But remember the usable DR is actually quite limited...
    Do you think so? My findings is that even very high contrast scenes (with limits, of course) will easily stay within the limits of the sensor's DR.

    Quote Originally Posted by Audy Erel View Post
    It's a constant battle between the noise floor and highlight clipping.
    Yes, of course. But my point is that battle will take place more in the realm of other light controls (lighting itself, aperture, ND, shutter speed), and during post, rather then by any change in ISO (either in camera or in post). Don't you agree?

    Quote Originally Posted by Audy Erel View Post
    We need to choose where to shift the middle grey according to our need/priority.
    I find this last sentence a bit in contradiction to the other ones. You you agree that changing ISO is like a A)"monitor brightness setting" and B)"It's all safely contained in the flat log image.", then what's the purpose of changing ISO?! Middle grey, more then anything will be set in post while color correcting/grading which, BTW, is done without the need to change the default ISO, right?
    Rui Guerra - PHOTOGUERRA Underwater Productions, Lda.
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  8. #8  
    The only time I have found adjusting the ISO up away from your base (like 800 ISO or whatever your base is) useful on set is in low light situations to allow the AC or operator to see the image better on the monitor to nail focus.
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  9. #9  
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    I think for good practical reasons there's a necessary place for 'ISO'-like adjustments in the image-capturing and processing chain.


    However, imo the further along in time we get with digital image capturing, the more anachronistic the old analogue film term 'ISO' becomes.

    It made sense during the initial transition from analogue to digital to use that sort of short-hand, familiar terminology that everyone sort of 'got' without having to give it too much thought. But that was then...


    I've mentioned this before, but I think the actual ISO (International Standards Organization) should create something like a Standardized Viewing LUT (SVL) for use with digital imaging systems.

    It would be a display LUT made by each camera manufacturer, for use in each of their cameras, set to a particular target determined by the ISO.

    It would be nothing more than a colour & gamma transform of the RAW (or other) sensor data, with positive and negative decimal values that the end-user can adjust like they currently do with 'ISO'.

    The only difference is, when set to SVL 0 (Zero), what you see in your monitor is the same thing (in terms of 'exposure') for every camera that complies with the SVL standard (much like analogue film was, or was supposed to be).

    Apart from the standardization itself, the benefit is, the terminology used makes it clear the SVL adjustments have nothing to do with your actual exposure but are purely how you're choosing to view that exposure.


    Obviously there's not much chance of such a standard being implemented, so the suggestion is a bit ridiculous, but imo, so is using the term 'ISO' in modern digital cameras.
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  10. #10  
    Senior Member Mark A. Jaeger's Avatar
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