Re: Linear Magnification

[T3D john bercovitz <bercov@xxxxxxxxxxx>]

P.J.Homer asks:

> If I use a supplementary lens on my 30mm Nimslo lenses which enables me 
> to get within six inches of the subject what is the linear magnification

Off the top of my head, that sounds like a tough problem.  Easiest to
measure at the time you're taking the picture.  (_That_ was helpful!)
I would think you would have to use the lens combination formula to 
find the combined focal length and then use other formulas to find the
primary and secondary nodal points.  Not trivial.  I'm trying to think
of what you might be able to do if you assume the Nimslo is focussed
at infinity (very approximately true) and so the front lens is operating 
at an infinite conjugate ratio.  That sounds like the direction to go in.
Hmmm...  OK, to the first order, with that assumption and some minor
assumptions, the magnification would be the focal length of the Nimslo
lens divided by the focal length of the covering lens.  So if the 
covering lens is 6" and the Nimslo lenses are 1.2" then the magnification 
is 1/5th (0.2).  Does this sound right?  I really shouldn't post until 
I've thought it through completely but it at least feels right.  8-)

John B


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End of TECH-3D Digest 165
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