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:

1/F=1/U+1/V

whereFis the Focal length of the lens,Uis the distance from negative to lens, andVthe distance from lens to paper.

and

d/D=U/V

wheredis the size of the negative,Dthe size of the paper,UandVas 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.