T3D Re: Revere lens board

[john bercovitz <bercov@xxxxxxxxxxx>]

Jon. is it really lens focal length you want to measure?  Does the 
lens board have focussing?  If not, I can see why you want to know
focal length: so that you'll know how much adjustment capability you 
need.

I just thought of another simple way to measure focal length.  Use
Netwon's form of the lens equation.  Then f = x'/m.  So just measure
the magnification of something and the distance you had to move the 
focal plane from the infinity focus, and you've got the focal length.
If you want to see how this is derived, you can pull down the paper
I wrote for SPIE form my web site.  See equa. 6a.

The amount of movement you'll need is x' = f^2/x.  This is equation 1a.

John B   http://www.angelfire.com/ca2/tech3d


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