What does the ASA/ISO do on a camera?

Question

I am in a first year photography class, and we use Nixon cameras with TRI-X 400 black and white film. The ISO is supposed to be set to 400, but I have no idea what ISO really does, and I accidently took my first three or four pictures with it on ISO 6400. Luckily I caught it soon enough, but what effect will that have? Is it possible those first few could still turn out?

Answer 1

If you expose the rest of the roll at ASA/ISO 400, the first few frames will be wasted. There might be the very faintest image, but since you will have the roll processed as if the whole roll was shot at ISO 400, the first few will be overexposed by 4 full stops. Tri-X can't handle this unless you push-process, but that would not work if the rest of the roll was shot at 400.

ISO or ASA is a measure of the film or sensor sensitivity to light. Low numbers mean less sensitive to light, but generally also mean sharper images. High numbers mean more sensitive to light, but generally mean somewhat less sharp images. This is one way that you can create different effects with your pictures - as you will learn in your class.