bellows length for 5x7 enlarger

[Brian Danieli]

how long does the bellows need to be to enlarge a 5x7 negative to 11x14 using a 180mm enlarging lens?

[Leonard Evens]

I can only give you an approximate answer since I don't know the true dimensions of a typical 5 x 7 image. Also, the aspect ratios are different, so you have to crop or reduce the size of the image, which would give different answers. Suppose you want to crop so as to enlarge 4.75 inches to 11 inches (a guess). That would be a magnification of about 2.32 times. The formula is that the ratio of the bellows extension to the focal length should be one more than the reciprocal of the magnification.

1 + 1/2.32 ~ 1.43

so the bellows should extend about 1.43 x 180 ~ 258 mm or about 10.15 inches.