Eye Piece Corrective Lenses

[Richard Clompus <rclompus@voicenet.com>]

Dear Fellow LUGnuts,

The topic of corrective eye piece lenses has come up.  Maybe I can help a
bit with this.  

Many new design SLR cameras come with a diopter adjustment built into the
eye piece. The R8 is a good example.  Pull out the tiny wheel to the let of
the eye piece, turn it to the correction you need and push it back in so it
locks into place.  Usually the corrective range is -2.00 diopters to +2.00
diopters.  If you are farsighted (hyperopic) you need plus (+) corrective
lenses.  If you are nearsighted (myopic) you need minus (-) corrective
lenses.  That's the easy part.  

If you have astigmatism, it means that the front part of your eye (the
cornea) is not a perfect spherical shape.  It is shaped more like the back
of a spoon or the side of football - but not as severe as this.  Most
things in nature are not perfect shapes.  Even under the best of
circumstances, it is hard to grind a perfectly spherical surface.  The
larger the lens, the more difficulty invovlved.  Best example of this is
the mirror in the Hubble space telescope.  Even a price tag of millions of
dollars couldn't produce a perfectly curved mirror.  NASA had to send up a
pair of "glasses" to correct the optics.  They never asked me to to make
this house call.  Interesting thought but probably not covered by their HMO
insurance ;-)  

With astigmatic corrective lenses, you must not only deal with the strength
of the lens but also the orientation.  Orientation is described as the axis
and is expressed in degress (starting at 0 and going up to 180.)  An
astigmatic prescription is then expressed as -1.00 diopter X 180 so that it
describes the strength (-1.00 diopter) and orientation (axis 180).  The
astigmatic eye piece corrective lens must be turned and locked into the
correct position.  If it gets turned even 10 degrees from where it should
be, your vision will be blurred.  Make sure that the eye piece is marked so
you know the final orientation on the camera.  This way the optician can
insert the lens so when you tighten the eye piece ring, it lines up the
proper way.

When you go to many commercial (Glasses In An Hour) optical shops, the
folks that work there can do some things very well and very quick.  But not
camera corrective lenses.  All spectacle lenses start out as a large (72mm
or larger diameter) and are cut down in a computerized edger to fit in your
frame.  The newest Japanese computerized edgers are fabulous and cost about
$28,000.  I have one at the office.  A high school student could run it.
It even gives you voice prompts.  The automated edging or grinding
instruments are not set up to make lenses 8-10 mm in diameter for eye
pieces.  So it must be HAND edged.  That means the optical technician must
be skilled and have an interest in doing this.  The optician must then
grind a lens from about 72mm diameter down to 10mm and get the orientation
correct when he pops it into the eye piece retaining ring.  It can be done
but I would recommend seeking the help of an older and wiser optician in
your area.  I'm 43.  If you could find an optician in his 60's, it would be
perfect.  If you can find a private shop or an optometrist who employs a
skilled optician, you are more likely to meet with success.  Its similar
that you can't go to McDonalds and get a prime rib steak. It is real
specialty work.  

I hope this has shed some light on a somewhat confusing topic.  

Happy shooting,

Richard
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Richard Clompus, OD
West Chester, PA, USA