General_T wrote in post #13153002
How do you do this for a camera and lens?
Some folks seem to think that what they see through the viewfinder (relative size of the object in the viewfinder as opposed to what they see with their naked eyes) would be the basis for a multiplier based on the focal length attached to their camera. That's wrong, though, because different cameras have different magnification optics in their viewfinder systems.
The most common method that would be used to come up with a magnification factor is to compare the focal length being used to the "normal" focal length (approximately equal to the diagonal of the film frame or sensor).
The bottom line is that I've never once talked to an experienced photographer who ever really considers an "X" power number for any lens.
If you want to compare two focal lengths, you'll find that the size of the image of a subject changes pretty much linearly with focal length. For example, if an object takes up 1 inch horizontally in a 4x6 print made with a 50 mm lens, then it would be roughly 2 inches wide if you shot the same scene from the same location with a 100 mm lens.