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

["Abram Klooswyk" <abram.klooswyk@xxxxxx>]

There seems indeed to be some confusion about loupe 
magnification. 

Lincoln Aug 16, 2000 :
>I am confused by loupes and their magnification.

Robert J. Vaughan Aug 19, 2000: 
>If one assumes that the average unaided viewing distance of 
>anything close up is 10 inches or about 250 mm, one can call 
>that unity magnification.  One can also assume that a 10 inch 
>focal length magnifier has (roughly) unity magnification. 
(...)
>Just divide the magnification into 10 inches or 250 (254) mm 
>to get the focal length of the magnifier.  Now, if you want 
>to get into serious magnification calculations, you can 
>consult optical texts but this simple formula works for most 
>situations.  
(...)
>The simple magnifier formula is: M=10/F for English system 
>and M=254/F for Metric. 

Peter Homer Aug 21, 2000:
>I was also interested in this reply, does this give what is 
>called "linear magnification".
(...)
>Although 250mm is generally quoted as the " Minimum distance 
>of distinct vision" or as you put it the "Average unaided 
>viewing distance for anything close up" this seems to be more 
>the limit of stereo convergence than focus.

The reason for this apparent uncertainty and confusion is that
the definition of magnification for handheld magnifiers is in 
fact not logical.
"Magnification" of lenses is well defined in optics, and its 
calculation is not very complicated.
But for the magnification of handheld magnifiers a different
definition has been adopted, based on an assumed normal 
viewing distance, an assumption not based on optical laws
and in fact very loosely based on the physiology of vision.

However, this definition of normal viewing distance is clear 
in itself, 250 mm is the distance. 
Not "about 250 mm" or "average unaided viewing" or "roughly", 
but just _exactly_  250 mm.  And not 10 inch but 250 mm.

"M=254/F for Metric" is confusing too, it assumes that F is
expressed in inches. That is not Metric!  More consistently:

Loupe magnification is 250 mm divided by focal length in mm.
That gives the exact magnification figure.

Or: 
Loupe magnification is 10 inch divided by focal length in 
inch. But that is an approximation, because 10 inch is not
250 but 254 mm.

These "formulas" only apply to loupe magnification and are 
not for general optical use, or for example for (macro) 
photography.

Abram Klooswyk