The theory is easy, the calculation is hard. Besides, I was bored and thought this might keep me occupied for a while. And yes, my numbers are for a 180mm lens!

The basic equations are:


where F is the Focal length of the lens, U is the distance from negative to lens, and V the distance from lens to paper.



where d is the size of the negative, D the size of the paper, U and V as above.

I just now realized that the 7" dimension of a 5x7" film isn't 7". That changes everything a little, but not too much. You need a little more extension, is all. If I'd had a negative with me I could have measured the image area, but I don't, so I can't.

Assuming 6 1/2" image area, it's
30.3cm neg to lens,
44.3cm lens to paper
74.6cm total.