I would like to copy some 4x5 negatives to positive. Everywhere I look for information about copying negatives to film, I find information about taking pictures of negatives with a camera. Using sheet film, it seems that I should be able to put a negative in contact with a sheet of unexposed film and make a positive directly. My goal is to be able to make backlit frames to display B&W and eventually color reversal images. How do I calculate the exposure? Am I trying to go about it the wrong way?