Re: [photo-3d] Digest Number 203 about magnifiers

["Robert J. Vaughan" <k0mz@xxxxxxxx>]

Hello Lincoln.

You had a question about magnifiers, magnification and focal length. 
There is a simple way to determine this relationship.  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.  Therefore, if one looks at something through a 5 inch
focal length lens, that would be 2X magnification.  A 2 inch or 50 mm
lens would have 5X magnification.  A 4X magnifier would have a focal
length of about 63 mm.  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.

A practical thought about what all of this can mean is that when the
focal length of the magnifier is the same as the focal length of the
camera taking lens, the magnified view of the film appears as an ortho
image and (for all practical purposes) exactly as the real subject
appeared to your eyes when you made the picture.  Does this help?  Now,
for all of us who have zoom lenses on our cameras, hmmmmm!; it gets
complicated.
Robert J. Vaughan

>    From: L <jet_lk@xxxxxxxxx>
> 
> >>I am confused by loupes and their magnification.  I am
> >>assuming that a 4X magnification loupe is equal to a
> >>specific focal length, but I don't know that for sure,
> >>or for that matter, what the focal length would be.
> 
> I think I should stick with a lens that has a focal
> length in the 40-60mm range.  I shoot most of my
> stereo shots at 50, but sometimes I zoom up to 70mm or
> out to 35.
> 
> Lincoln
>