Shooting at native and derived ISOs

[Dave Blinder]

Maybe this stuff is common sense, but came as a bit of surprise to me. I often shoot at 1/3rd ISO increments on my 40D and 7D, but maybe I shouldn't...

http://shootintheshot.joshsilfen.com/2010/05/13/canon-hd-dslr-native-iso/

[Roger Clark]

That is another partly misguided article. What is important in the perception of noise in images is signal-to-noise ratio (S/N), not simply noise level. So if a digital camera were at ISO 125 and multiplies the digital numbers to boost the signal from ISO 100, both the signal and noise are boosted, so at best no change in S/N. Similarly at ISO 160, the signal is reduced so signal and noise would at best be reduced equally so S//N stays the same. But it is more complex. Emil Martinec has done some analyses on specific cameras (perhaps he can chime in). On some canon cameras, a second post amplifier is used for the intermediate ISOs and that amplifier adds a small amount of noise, but this is noise seen only at the noise floor. A couple of stops above the blackest black, you can't tell the difference. If I remember right, some Nikons he tested do not have this problem so there is not a disadvantage in using intermediate ISO on those cameras.

Personally, if I need an intermediate ISO I choose the next highest multiple of 2 (from 100) ISO and use exposure compensation to underesxpose. The advantage here is exposure sets the number of photons collected by the sensor, and that is independent of ISO. If you use the higher ISO setting, the noise floor is lower in terms of photons and by using exposure compensation, you don't sacrifice dynamic range at the high end. But often instead of exposure compensation, I select the ISO (again multiples of 100) and set exposure and aperture manually.

Roger