[photo-3d] Re: Better Viewers

[Dylan the Hippy Wabbit <dylan.wabbit@xxxxxxxxxxxx>]

Hi All,

Mike Galazin asked:

>If one can use two achromats together to achieve half the focal length of
a
>single one, then what would you get for focal length with three achromats
>together?  For example, I can get a half dozen 38mm diameter achromats
with
>140mm focal length; would three together give you half of half of 140,
i.e.,
>35mm FL? 

Actually it would come to a third, or 46 2/3 mm.

This sort of calculations are actually easier if you work with the lens
power in dioptres.  That is one metre (1000mm for those whose heads are
stuck in obsolete units  ;-)  ) divided by the focal length.  The advantage
is that lens powers can be just added together.  So the 140mm lenses have a
power of about 7.14 dioptres, and two have a power of 14.28.  Three come to
21.43.  Easy!  Then divide the power into 1000 to get back to famiar focal
lengths.

Bear in mind though that the more glass you put into your viewer the more
absorbtion/refletion/lens imperfections are going to add up.

Hope that helps.
-- 

Dave Spacey

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