[ATM] Determining the focal length of a negative meniscus lens.

["Thomas Janstrom" <tomjan@ozemail.com.au>]

Hi All, 

OK, this one has me stumped, how do I measure the focal length of a
negative meniscus lens? Richard mentioned using a biconvex lens of a
known FL to "pre-collimate" the light source, but I don't see how this
would work, unless I had a lens of the same FL to use as the one under
test.

I've gone through what books I have and they for the most part only deal
with the testing of positive lens and they only "touch on" negative
lenses and then they are not of the meniscus type anyway, before going
back to positive ones. One thing I did come across was their (neg
meniscuses) use in early achromatic objectives (read 1880's or early).

I can build a test bench easily enough, but that's not really the
problem, I just don't know how to work with negative lenses, never
having had much to do with them in the past.

I have light sources for the following lines (well near enough, at most
they are only a few nm out), C (long by 3.6nm), e (long by 21nm),
F'(short by 9nm), d (exact freq'), and g (short by 7.8nm). I'm hoping
that these should give me enough info to peg the glass type used in the
lens that I have to a reasonable degree of accuracy. For more info on
these lenses see the thread on "reverse engineering a lens".

So there we go, any help with this little conundrum would be great!

Thanks all.

Clear Skies, Thomas.
www.tjanstrom.com
www.norsewines.com.au 

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