How you calculate the magnification of a telephoto lens?

Answer 1

Step 1

Write down the maximum focal length of the lens for which you want to find the magnification. For instance, a 70-250 mm lens will have a maximum focal length of 250mm. A prime lens will have only one value listed, and this value should be considered the maximum focal length.

Step 2

Multiply this value by 1.6 if you are attaching the lens to a digital camera. Most digital cameras have a 1.6x "crop factor", which means that the outside edges of a scene will be cropped because they do not physically fit on the sensor, making the effective focal length 1.6 times greater than it would be on a film camera. For a 250mm lens, the effective focal length would be 400 (250 times 1.6).

Step 3

Divide the effective focal length by 100. An easier way to do this is to move the decimal point two spaces to the left. The effective focal length of a 250mm lens on a digital camera is 400mm, so divide 400 by 100 to arrive at four.

Step 4

Multiply this value by two to find the magnification of the lens in terms of viewfinder magnification used in binoculars or telescopes. So a 250mm lens would have an 8x magnification (400 divided by 100 multiplied by two) on a digital camera.