I've got a couple questions regarding film speed. I'm mostly concerned with black and white film, but I'm sure some of this applies to color film as well. First, what is the difference between ISO and EI? Second, let's say I'm using a 400 speed film, and I want to push it to 1600. What changes? I assume I'd change the ISO setting on my camera to 1600, but what changes in the development process? Is there a general rule of thumb for figuring out developing times, or is it more or less guess and check?