Hi guys I have a question. In analogical cameras you have the ISO adjustment, for example, if you udse a 400 ISO filme you set your camera to 400 ISO, but what happends if I set my camera with 800 or 100 ISO with a 400 ISO film???
You got lots of good answers here. Key takeaway is that the ISO dial does not cause the types of changes it does in digital -- it merely calibrates the built-in meter to give you (more) accurate exposure settings. Cameras without a built-in meter don't have an ISO dial!
If you load 400 speed film and set the ISO dial to 100, and follow the meter, you'll overexpose your pics by two stops. If you set the dial to 100, then shoot using the "sunny 16" rule and ignore your meter, the photos will be fine.
When I am shooting in low light, I often load 400 speed film and set the ISO dial to 1600, intentionally under-exposing the film by two stops. I then compensate by developing the film for longer -- this is push-processing. The photos are properly exposed but grainer and more contrasty. The key is altering the development; if I developed normally, I'd have underexposed (dark) photos.
Aaron