Magnification is normally used in photography to mean the ratio
between object size and image size.  This is not what you seem to mean.
I think the question is about making the photograph look the same
size to the eye as the original scene did from the camera position.

The image in a photograph will look the same size as the scene did
from the camera position if:

viewing distance = focal length x englargement magnification

(Technically it should be the distance between the film plane and
the rear nodal point of the lens instead of focal length, but using
focal length is a good enough approximation except for extreme
close-up photography)

So if you use a two inch lens and print a 5X enlargement (5x7
inch print) the image in the photograph will look the same
size as it did from the camera position if you hold the picture
10 inches away.

If you use a 300mm lens (about 12 inches) and also make
a 5x7 inch print it will appear 6x life size if held 10 inches
away. If you look at the print from five feet away, the image
will look the same size as what you saw from the camera
position. If you look at a 1x1 1/2 inch contact print from
one foot away the image will also look the same size as the
scene you saw from the camera position.

If you enlarge the image from the negative taken with the
2 inch lens to 11 x 14 inches (11 times enlargement) the
image will look 2.2x the normal size when viewed from
10 inches away, and will look normal sized when viewed
from 22 inches away.

I hope this helps.

Peter.

  While lens magnification is not generally used in photography you can
assume roughly that 50mm equals 1:1. in 35mm. But this really is about the
apparent image size in a theoretical camera as many cameras these days have
a smaller image in the viewfinder to save size/weight  and to increase
brightness.
   If you wish to be fussier with the math, then a 43mm lens would be 1:1
in that same theoretical slr.
   Remember that this has only to do with apparent image size when viewing
through the camera.

   Either Peter is not awake or he got up on the wrong side of the bed, or
maybe he is some kind of language nut.  Your question was just fine and
clear.  Your use of the term magnification is acceptable. The rest of the
world uses it that way.

   I won't bother to repeat what Tony wrote as he did a good job.

       If accuracy is not paramount, then this works fine. A 500mm lens
provides roughly 10X magnification.

       This runs into lots of other problems if you're trying to be
accurate, though, among them "true" focal length (which often disagrees
with what the lens is sold as), focal distance, and viewfinder
magnification.

    - Al.