[photo-3d] Re: Loupe question, magnifiers, confusion

[Abram Klooswyk <abram.klooswyk@xxxxxx>]

Mike Davis 05 Nov 2000 :
>It turns out that the magnification offered by a lens varies 
>with the close focusing abilities of the user.  

This adds to confusion, in my opinion :-). 

In the original thread, which was from 16 to 23 aug 2000, 
Lincoln Stamm 16 Aug 2000 asked (16 Aug 2000, subject: another
loupe question):
>I am confused by loupes and their magnification.  I am
>assuming that a 4X magnification loupe is equal to a
>specific focal length (...)

So the question was about the indication of the magnification
of a lens used as loupe.

I wrote at that time: ...for lenses which are hold close to
the eyes, like glasses, loupes or magnifiers, stereoscope
lenses and eyepieces of all kinds of instruments, a different
magnification definition is used, depending on the arbitrary
"normal" viewing distance of 250 mm.
... an assumption not based on optical laws and in fact very
loosely based on the physiology of vision.

So _by definition_ loupe magnification is 250 mm divided by
focal length in mm.

When you vary distances and focussing of the eyes of course
the effective magnification will differ. However, the original
question was not on perceived magnification but on loupes.

Abram Klooswyk