I should have added that the 10"/250mm number is also an important
number for depth of field computation. When determining the required
limit of resolution of the human eye for the value of "Circle of
Confusion" the eye is considered to be at normal reading distance when
viewing an image.
Chuck Norcutt
Chuck Norcutt wrote:
> 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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