taylorivens

Hi everyone, I am a college student currently taking an Introduction to Photographic Medium class and I have a few questions about processing film and how the camera works. I have a Canon EOS Rebel X and I use 400 TX film. 1. Some of my negatives on one roll may come out perfect meanwhile some do not come out at all. Why is that? 2. How does contrast play a role in the overall picture? and how do you produce contrast?