Choosing an ISA
Ok choosing one outdoors is pretty easy. If it is sunny outside, choose 100 ISA or 200 if that is the lowest the camera offers. You can also put it on a sunny mode if your camera has it.
If it is very cloudy, you could stick with 200.
Now if your inside, and you only have the camera flash, do you put it on 400, and 200 if you have an external flash if your subject is within 10 feet? Will the camera usually compensate on auto setting if it knows you have a external flash? With a higher ISA it means less light needs to hit the lens in order to use a smaller aperture setting. Don't know why it's backwards, smaller aperture should mean the opening is smaller, but it's the other way around which confuses us vacation type of photographer.
When would you use a higher setting over ISA 400? I would think higher ISA would allow a faster shutter speed?