AE compensation versus ISO

[calistoga]

When you have a low light condition, which is better to adjust? Do you adjust one at a time or both?
Thanks.

[Digidave]

That really depends on the situation. If you have a stable(nonmoving) subject, Exposure Compensation is the way to go. You will get slower shutter speeds but won't have to worry about subject movement. You will though, have to worry about camera shake. A tripod would solve that problem. If you have fast moving subjects, you really have no choice but to raise your ISO. This will allow for faster shutter speeds.

[Doug Pardee]

Originally Posted by calistoga

When you have a low light condition, which is better to adjust?

They're not at all the same.

Changing AE Compensation will make your photo lighter or darker by making adjusting the exposure relative to the ISO.

Changing the ISO will not make your photo lighter or darker, it just changes the standard exposure.

Example:

Start at ISO 100, f/8, 1/100th of a second.
ISO 200 in A/Av mode gives ISO 200, f/8, 1/200th of a second.
ISO 200 in S/Tv mode gives ISO 200, f/11, 1/100th of a second.
+1Ev of AEC in A/Av mode gives ISO 100, f/8, 1/50th of a second.
+1Ev of AEC in S/Tv mode gives ISO 100, f/4, 1/100th of a second.

All five approaches give different settings. The first three will all have the same overall brightness. Only the tradeoffs between ISO, aperture, and shutter speed are adjusted. The final two will be one stop brighter than the others.