Re: Eye Piece Corrective Lenses

[Gerard Captijn <captyng@vtx.ch>]

At 10:29 AM 29/08/1997 -0400, you wrote:
>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
>----------------------
>Richard Clompus, OD
>West Chester, PA, USA
>

Thanks Richard for your very interesting information. And another great
example of the type of message that makes the LUG a really substantial and
useful forum for Leica photographers.

Gerard Captijn.