David,
Here is a way to find the focal length of your eyepieces by direct
experiment that I posted a while back. It worked very well for me.


"If you want to try measuring the focal lengths of your eyepieces here is

a simple way that seems to give me good accuracy of about +-0.1 mm in
the focal length even for the short focal length eyepieces. It also
works if the eyepiece is used with a Barlow and gives the effective
focal length of the eyepiece+Barlow combination so you can thus get the
amplification factor.

Its based on measuring the magnification seen when you use the eyepiece
to project a Ronchi grating on to a distant screen (im my case a white
wall). To use it you put a Ronchi grating behind the eye end of the
eyepiece, with a flashlight behind the Ronchi to illuminate it. You then
carefully adjust the separation of the eyepiece from the Ronchi so that
a sharp (greatly enlarged) image of the grating is shown on the wall.
You the measure the separation of the projected lines on the wall (I
measure the span of about 5 cycles and then divide by 5). Knowing the #
of lines/inch on the grating you divide the separation on the grating
itself into the measured separation as projected to get the
magnification. The only thing left is to measure the separation of the
eyepiece from the wall. To a first approximation the eyepiece focal
length is that distance divided by the magnification. If the distance to
the wall is much larger than the focal length the error will be small.
To get a better approximation you have to allow for the separation of
the eyepiece principle planes. You can correct for that by changing the
distance to the wall and repeating the measurement. The math for that is
not hard but perhaps a bit long for a post here. By the way I posted on
this in sci.astro.amateur back on about Feb 10,2000.

If you don't have access to a Ronchi grating, a transparent
ruler with a mm scale can also be used but the projected mm separation
is large for short focal length eyepieces and distortion may then throw
off the result.

Have fun if you try it,
Don Taylor"




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