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Got a 10x Horizon loupe for my Arca Swiss. Somehow I assumed a 10x loupe would m agnify things, well 10 fold. When I look at some horizontal pattern (pair of lin es for example) through the loupe with my right eye and at the same time without loupe with my left eye, it appears about 2 to 3fold larger, no more. Is there s ome optical law I am missing???
It can be hard to judge the magnification factor just by looking. One way to check would be to compare the loupe with others of various magnifications.
I have seen one quantitivative definition of loupe magnification. It is based on the idea that the way a loupe magnifies is by allowing your eye to focus closer to an object than normal. The normal closest unaided viewing distance is assumed to be 25 cm (10 inches). The magnification is then 25 cm divided by the focal length of the magnifier. So if you can estimate the focal length of your loupe, it should be about 2.5 cm (1 inch).
For what its worth, a x10 loupe might not be the best choice for viewing the ground glass image. For checking focus on the ground glass, my opinion is that the optimum magnification is about x4. Because of the grain of the ground glass, higher magnifications don't reveal additional detail. They do make a dimmer image.
Thanks Michael, you are right of course - a loupe does NOT magnify, it let's the eye focus on a closer distance. Checking my 10x Horizon, I can see any object through it without eye-strain very clearly - if I hold the same object about 10 inches away (i.e. no eye-strain without loupe), then indeed it appears about 10 times smaller than with the loupe!
I actually do find the 10x easier to focus with than the 3.6x Toyo loupe I also have. Guess, it all depends on the particular ground glass you have and how the fresnel is cut...