Originally posted by azerak If I use a 400 ISO film and I changed the ISO on body to 800, what I understood is that it might be underexposed. Seeing that I am most probably going to scan them rather than printing them, it is OK because I can always edit later in post processing, right? Or it won't make a difference later?
It depends on B&W or color, and for color slide or print. For B&W or E6 processing the best thing to do is increase the processing time to compensate for the under exposure.
Labs can do this for you or if you process your own then you can do it. Color print film ( like C41) is harder to deal with.
I have shot B&W for over 30 years on and off and only rarely shoot at the recommended ISO