Quote Originally Posted by jsimoespedro View Post
As far as I now, when you over-expose or under-expose, you also correct the development time.
While, yes that is suggested in many instruction sheets and other sources, it is by no means required. It also doesn't change the ISO film speed.

Most sources also admit there is a corresponding loss of detail (on film) when exposing at, for example, 200 instead of the box speed of 100.

The zone system and its variants adjust development to match measured scene contrast to a given paper grade.

Another option though is simply to develop the film normally and then adjust contrast with the paper grade.