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Old 9th September 2013, 02:07 PM
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RobF RobF is offline
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Originally Posted by peterh View Post

A shallow depth of field (which is what you want for rangefinding) is determined by the focal length (the 50 in 10-50x) and the size of the lens opening (the 60 in 10-50x60). Which has nothing whatsoever to do with the eyepiece, and more with that huge bulk of glass at the front of the scope.

This is not entirely true... the scientific explanation has baffled many, including some professional photographers... but for all practical purposes, to the eye, the larger the magnification, the smaller the depth of focus.

The effect on the depth of focus of the lens opening is hard, and scientifically correct. The wider your lens opens, the shallower the depth of field.
True, but what I am saying is the actual mag number, the 50x is a figure derived from the focal length of the objective divided by the ocular. So if scope A has a focal length of the objective of a 1000mm and the ocular of 20mm it's a 50x, but that can be the same mag as scope B which has an objective FL of 2000mm and and ocular of 40mm... they perform differently despite having both quoted mags of 50x..., the latter scope B having shallower depth of field at the same mag, even though the objective sizes are the same, because the focal lengths of the lens clusters are different.
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