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Is your camera ISO invariant?


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A discussion about noise at indoor sporting venue lead to the question: "is your camera ISO invariant?".

Definition: shooting initially at lower ISO and then boosting the exposure in post will result in the same noise as if the image was originally shot with high ISO.

It sounds like a good thing: shoot with high ISO and get the exposure correct in camera. But there is one problem, not all cameras have that property. I did some testing with my bodies A7RIV and A9II, and the results were not the same. After some research and reading testing results on DPReview, I redid my tests with the assumption that my bodies are not ISO invariant.

The result lead to the conclusion that my bodies are definitely not ISO invariant. The A9II was kind of a given sine it is a stacked sensor, and that property are not used with stacked sensors. But I was surprised the result was the same for the A7RIV, which is not a stacked sensor.

The finishing is, sensors with dual gains and not ISO invariant. The A7RIV has a second gain around ISO 320-400. The A9II on the other hand has a second gain at ISO 800.

Let me give you a simple setting for the venue shooting basketball. Shutter speed : 1/1250th , f/2.8. With these 2 choices, the ISO needed to be at 6400 for this venue. I made the first shots at that ISO, and then again at the second gain ISO and boosting 3 stops in post. The images, as concluded by DPReview, were less noisy at second gain ISO.

What about you camera? Is it ISO invariant, or does it have a dual gain sensor?

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This is fascinating and so very helpful. It just shows that each camera body has its own unique properties. 

For almost a year I thought mine were ISO invariant, but I was shocked to find they were not. There was a discussion on DRO that didn't sit well with me, and that lead me to this result. I will make a thread on that subject and the brightness of the viewfinder soon.

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