I have been reading Roger Clark's article on sensor performance, in particular his comments on "unity gain" (
here).
His statement that "Since 1 electron (1 converted photon) is the smallest quantum that makes sense to digitize, there is little point in increasing ISO above the Unity Gain ISO (small gains may be realized due to quantization effects, but as ISO is increased, dynamic range decreases)." has got me thinking about the super high ISO values that people often mention when taking nightscapes.
If Roger's statement is correct, what is the benefit (if any) to using an ISO value of say 3200, 6400 or even 12800 if the unity gain value is much much lower?? Is it simply that the image doesn't need to be stretched as much in processing, or is there more to it than that? A genuine question, as I am tossing up whether I need to upgrade my camera body sooner rather than later, or if I can spend the dosh on new lenses instead.
My current daytime camera (a 400D) has an ISO range of 100-1600 (or up to 3000 if i use a firmware hack), however the unity gain ISO is ~1220. So where am I losing out by shooting at ISO1600 in comparison to a new camera (say a 5DII) which has a maximum ISO of i think 25600 yet has a unity gain ISO of about 400??
