What's APX? What's ISO? Are they related? How does one measure them? I was told I got ISO 100 BW film... now I figure out it's ISO 400? Minor details like that can really make my day. The actual roll looks like this: http://andrew4137.fotopic.net/p16217715.html On the box it says Agfapan APX 135-36 So, what's the ISO speed of my film?