To find the Camera Extension necessary for a given magnification.
Multiply the focal length of negative lens by magnification less 1.
Example. Magnification 5 and negative lens 3 inches, then 3 X (5-1) = 3 X 4 = 12. Therefore, 12 inches would be the camera extension.
It will be apparent that a shorter focus negative lens would need less camera extension for the same number of magnifications, but, although this is the case, the shorter negative lens means smaller covering power, and the plate might not be covered to the margins with a low magnifying power.
To find the magnification possible with a given camera extension.
Divide the camera extension by the focal length of negative lens and add 1.
Example. With a camera extension of 10 inches and a negative lens of 1 1/4 inches, what magnification can we obtain ?
To find the focal length of the complete lens.
Multiply the focal length of the positive lens by the number of magnifications.