ISO Invariance for RAW

[Saikat Chattopadhyay]

Hi,

I am posting here for the first time, so little bit nervous. The topic might not be properly linked to aesthetics or techniques of Cinematography . Kindly let me know if this is not suitable subject (or has been discussed before).

I was trying to dive into the term ‘ISO Invariance’ and I came across some videos and articles (linked below) which confused me even more. My understanding(might be incorrect) is that the voltage at pixel level ,when the photosite gets exposed to light, is very low and the Analog value is amplified by some factors then sent to the ADC,followed by De-Bayering, Chroma Subsampling etc. The gain of the voltage here should be linked to ISO we set and the factor will differ as we play around. 
Now for the RAW data we do have control in post to change the ISO , which implies that ISO is not a deciding factor like Color Temperature and not mapped on the footage.However, in the videos and articles I see that the tests are showing Photographic Cameras are not totally ISO invariant and they are introducing noise when set to higher ISO.

Unfortunately I don’t own a high end Cinema Camera to test this and didn’t find anything related , so my question is :

are cinema cameras really ISO invariant?

If yes, should we care about setting up proper ISO on set ? If no, what’s the point of having an external control in post to modify it?

https://photographylife.com/iso-invariance-explained

 

Thanks in advance. ?

 

[Nicolas POISSON]

Although not a pro, I have made a lot of research about dynamic range, ISO, and all that stuff. Here is a summary of what I understood.

Basically, ISO is gain. But this gain is the multiplication of analog gain at the sensor output – and before ADC – and digital gain. If analog gain is fixed, then ISO is simply digital gain and the camera is ISO-less. It means ISO setting changes absolutely nothing when shooting RAW still images in a DSLR. It just increases the brightness of the image displayed in the electronic viewfinder or on the rear screen of the camera. However, even if the camera is ISO-less, ISO setting does affect jpeg and H264 video.

Some cameras have variable analog gain. Changing the analog gain does affect RAW. I am not aware of any consumer camera that allows to explicitly configure analog gain and digital gain. Some cameras allow this indirectly. This is what the “DR” setting does on Fuji X cameras. Compared to “DR100”, DR200 halves the analog gain, and doubles digital gain. The overall gain is the same (hence same ISO), but it is less before ADC and more in “post processing”. It is useful in the situation where the sensor is not clipped but the ADC is.

Why not simply setting a lower fixed analog gain? I guess it is a question of image quality. If one want to increase the gain, it could be better to first raise the analog gain a bit, and then raise the digital amplification. It suggests that there is no clear winner in the battle between analog vs. digital gain. It seems analog gain is better if applied first, but in a small amount. If one want to go further, it is better to switch to digital.

https://www.dpreview.com/forums/post/58627880

There is another technique called “dual native iso”. I am not an expert of sensors. My understanding is that a sensor integrates onboard components to read the signal. Driving these components at higher or lower voltages modify both the sensor noise and its ability to accept a lot of photons. The dynamic range does not change. If one reduce the noise (better low light performance), one reduce the sensor clip threshold by the same amount. This is useful in low light + low contrast conditions, where no pixel receives much photons anyway.

Concerning “high end cameras”, ARRI uses another technique, reading each pixel of the sensor with two different circuits in parallel (simultaneously), with different analog gain, combining the two images to produce a single higher dynamic range image. Canon does the same in some cameras.

https://www.arri.com/en/learn-help/technology/alev-sensors

https://www.newsshooter.com/2020/06/05/canons-dual-gain-output-sensor-explained/

“dual native ISO” does not increase dynamic range, while “dual gain” does. A “dual native iso” camera either use one or the other ISO, not both at the same time. ARRI’s technique is closer to some kind of “exposure bracketing”, like if you were taking two shots of the same scene, at the same time, using one shot with the sensor optimized for low light, the other for high light. A camera with dual native ISO is not ISO-less, while, if I understand correctly, the Alexa or the C300 are.

http://neiloseman.com/alexa-iso-tests/

 

Thus, for your question, of whether setting ISO is important, I’d say :

1- it is definitely important on non ISO-less cameras, and not all cameras are ISO-less.

2- even on an ISO-less camera, you would set your exposure (time, aperture) according to what you see on your monitor. If your monitor is off because of weird ISO settings, you may record poor footage choosing wrong aperture or exposure time. Setting the ISO will not change the dynamic range, but it will dictate how much you open the aperture, which in return dictate how much stops you allow over and under “middle gray”. What highlight you keep, what noise you get.

Edited July 5, 2020 by Nicolas POISSON

[David Mullen ASC]

Whether or not a camera is ISO invariant, giving the sensor less exposure and then compensating by adding gain / brightening in post will lead to noise. It's just that some cameras at higher ISOs will add more and more noise reduction to compensate.