When a camera lens is focused at infinity, the rear principal point is exactly one focal length in front of the film. To find the front principal point, take the lens off the camera and let light from a distant object pass through it “backwards”. Find the point where the image is formed, and measure toward the lens one focal length. With some lenses, particularly ultra wides, you can’t do this, since the image is not formed in front of the front element.
So object to front principal point distance.
Si rear principal point to image distance
f focal length
1/So + 1/Si = 1/f
M = Si/So
(So-f)*(Si-f) = f^2
M = f/(So-f) = (Si-f)/f
If we interpret Si-f as the “extension” of the lens beyond infinity focus, then we see that this extension is inversely proportional to a similar “extension” of the object.
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Tags: 17, 18-55, 35-70, 55, 80-200, front, frontal, nikkor, nikon, optical