If increasing the gain also increases the noise, what's the purpose of gain, when there are 2 other variables (aperture and shutter speed) that can alter exposure?
@merrylewang You can play with all three variables to get the best subjective image because all three have pros and cons. Getting more light using a longer shutter speed increases motion blur, using a different aperture can change DOF and the aesthetics of your image, and using a different gain increases noise (as shown here).
ISO increases exposure by increasing an image's sensitivity to light. So the amount of light caught is the same but the sensitivity different as opposed to increasing the aperture size and catching more light in this case you are instead increasing the sensitivity to the already cached light.
What would be the point of really ever going past ISO 400 or 800, let's say? I've heard the general rule of thumb (could be totally wrong) when shooting photos to stay within ISO 800, and to adjust all the other controls at that point.
@c-chang I think if you are capturing a scene with high probability of motion blur, like a fast paced sport or birds flying, you would need a higher ISO, since the shutter speed speed is going to be really low for a sharp image. The tradeoff would be grainy noise probably.