To calculate the magnification of a photograph taken on a microscope fitted with a 35mm camera:

 (photo eye-piece) x (camera factor) x (objective) x (enlargement)

 e.g. For Zeiss Axiovert: 10 (fixed) x 0.25 (fixed) x 40 x 3.5

The enlargement convention is based on linear measurement not area thus is in this case the photo length (or width) divided by the negative length of 35 mm (or width of 24mm)