maybe not such a trivial question, but I am unsure about the following:
it is common knowledge especially in underwater shooting (all manual, no such thing like TTL) that the flash exposure at a given flash strengths is controlled by the aperture and thus affecting the foreground, while the ambient light (background, environment) is controlled by the shutter speed.
If I have a setting to start e.g. f8 and 1/125 sec at ISO 100 and by this my foreground is well exposed but the background is too dark, then I could go to 1/60 sec. If for some reasons I do not want to change the shutter speed but want to increase the ISO from 100 to 200 would that change my ambient exposure of the background AND also my flash exposure of the foreground? Or only the background by 1 full stop? or both by half a stop?
It is clear to me that exposing without flash a f8 1/125s at ISO 100 configuration would yield the same as f11 1/125s at ISO 200 or f8 1/250s at ISO 200 (that is a 1 stop difference each). This is not regarding DOF by the changing aperture or freezing effects by the changing shutter speed, just for the exposure value.
But how is it with the flash?
let's assume I have a good exposure for background and foreground with settings like f8 1/125s at ISO 200 with a half power of the flash. What is the aperture and shutter speed if ISO is changed to 100 (same flash power) which gives me the same overall exposure than in the beginning?
thanks for enlightenment!