Re: testing focal length question

[T3D John Bercovitz <bercov@xxxxxxxxxxx>]

> how does the distance between the two elements
> affect the image sharpness and magnification and overall quality?

Generally you want to measure over the outside of the lens
to get a distance from the front of the front surface to the
back of the back surface.  Even front cell focussing triplets
are best at one separation.  Some lens designs are more sensitive
than others to the spacing.  I will have to see if I can find
anything on the Rapid Rectilinears.  I know some lenses want to
have their spacing correct to 0.1 mm (.004").

You could also try different cells - mix and match and check the
resolution but that could be a large project.

John B


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