Moose's reference is interesting and I'll have to study it a bit. But
the simple formula I used is something I picked up from an Edmund
Scientific book on optics probably 35 or 40 years ago. They used a
simple definition for the power of a magnifying lens as 10 inches
divided by the focal length of the lens in inches. I simply converted
inches to millimeters and rounded 10 inches to 250mm. I believe this is
the formula you'll find used for a jeweler's loupe. A one inch lens is
considered to be a 10X magnifier.
Actual magnification depends on the relative positions of the eye, lens
and subject but I believe you'll find that 10" or 250mm is used as a
standard since that is the accepted normal distance of the eye from the
printed page for reading.
Chuck Norcutt
Moose wrote:
> Dan Mitchell wrote:
>> chucknorcutt@xxxxxxxxxxxxxxxx wrote:
>>
>>> A Zuiko 50/1.8 (mount end toward film) makes a superb 5X loupe.
>>>
>> > For magnificaton of other lenses as a loupe divide 250 by focal length.
>>
>> Just curious -- where does 250/length come from? Why is 250 the magic
>> number here? It doesn't seem to be a number that I'd expect to "fall
>> out" of optics, but I must be missing something.
>>
> http://en.wikipedia.org/wiki/Magnifying_glass
>
> The formula Chuck gives is a simpler version of the second formula in
> the article. It combines the diopter and power calculations into one.
>
> Moose
>
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