[photo-3d] Re Digest Number 204 about magnifiers

[Peter Homer <P.J.Homer@xxxxxxxxxxxxx>]

Robert J. Vaughan wrote in reply to Lincoln's post

>
>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.

I was also interested in this reply, does this give what is called "linear
magnification" I am in a folio which has a box for this on the folders used
but no one in the folio seemed able to explain to me how to calculate it
for the close ups I occaisionaly enter. I did get an explanation from some
one on Tech-3d. Although 250mm is generaly 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.
This is why people shut one eye to thread a needle and personaly I find
that I can Focus as close as 190mm like this but I appreciate that normaly
we use two eyes for viewing        P.J.Homer