hi kruger

each stop doubles or halfs the light going into the camera hitting the film.
changing your shutter from 60 to 30 doubles the light, changing from 60 to 125 halfs the light
just like going from f8 to f5.6, and f8 to f11 ... manually adjusting the asa/iso value that is connected to your camera's meter
will do the same thing ... going from asa 400 to 200 doubles the light, asa 400 to 800 halfs the light ...

adjusting the iso or fstop gives you more control on how much you over or under expose your film, since your camera can't
go in - between shutter speeds ...

you will have to have the lab over or under develop the whole roll, it can't be done after the fact ...
you can over and under develop single frames on the roll, just by how you expose it ...

sometimes the best thing to do is to shoot a few rolls, and "bracket" your exposures so you expose 1 shot right-on what the meter says,
one with 1 stop more light, and one with 1 stop less light so you can get an idea of what your lab ( and camera )
is actually doing before sending them anything important ...

depending on the film you are using, sometimes the makers don't recommend anything but processing "normally" with 1 stop over or under exposure
because the film has 1 stop leeway built in to it ...


john