Tim Clark wrote:
>> Lots of us are using an extra 50/1.8 as a loupe. Anybody have any idea
>> approximately what magnification ratio this lens would have when used as a
>> loupe?>>
>Well, we've heard 5X and 8X so far. Is there a definitive answer to this
>question??
Well, I don't think there is a definitive answer. If you check the
following page you'll discover many jeweler's loupes for sale. All of
them conform to my original rule that the magnification is the focal
length in inches divided into 10 (or the focal length in millimeters
divided into 254). I did, however, also discover another site where a
3.6X loupe was listed as having a 60mm focal length which would assume
216mm instead of 254mm. I don't think it's a very precise measure.
Your actual results depend on the ratio of the apparent size of the
image with and without the lens. That depends on how close you can
focus your eye. It all points back to my original post that said it was
based on 10 inches because that was "normal" reading distance. "Normal"
is a bit different for each of us. My 5X lens might be your 4.5 or 5.5X
lens.
see...
http://www.omgems.com/jewelold/loupes/loupes.htm
ps: One of the pages pointed to earlier described how to build a loupe
with a transparent plastic base. The instructions were fine except that
the author was confused about loupe magnification. He couldn't
understand why his 50mm lens seemed to give more magnification than his
75mm lens. He did not understand some basic optics. On the camera the
75mm lens projects a larger image of a relatively distant object than
the 50mm lens. But as a hand magnifier the shorter lens produces
greater magnification.
Chuck Norcutt
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