Oh my Lordy, Rich...I avoid getting into astigmatism discussions as it seems
to be one of the most difficult topics for those online to understand. I can
tell you some truths:
Greatly Shortened Astigmatism 101
1) Astigmatism won't really affect your focusing "terribly" until you get up
to about a power of 1.50 and up. Below that and the muscles of the eye can
usually compensate for the short duration of each focus. (I would suggest
strongly that you have astigmatic correction for night driving if your
astigmatism is greater than 1.0...we tend to be more fatigued at ngiht and it
will affect you more and blur the distance.) But for many, camera focusing is
not affected until after 1.50. Each person will have to determine this on
their own.
2) Astigmatism will not really affect your focusing, and you will be able to
use an eyecup or diopter correction IF your astigmatism is on either the
vertical or horizontal axis (near 90 (vertical) or near 0 or 180 (the
horizontal axis). Only if your astigmatism is on an obique axis (near 45 or
135) and over 1.00 dopters will you likely have a problem using diopter
correction.
It was said:
<<It seems that astigmatism causes the addition of a dioptor correction lens
to
the viewfinder of an OM camera not too be effective.>>
As you can see from the above statements, this statement is not true. The
writer above was simply due for a new prescription and his astigmatisn may
have grown to the point where it was greater than 1.0 and therefore needed it
to be ground to correct the path of focus. Those with less astigmatism can
certainly get by with an eyecup. I have also seen those with 2.00 of
astigmatism, but the axis was at 90 (vertical), so they could focus fine with
an eyecup.
Another stated:
<<Adding prisms to correct for astigmatism
in the viewfinder isn't all that feasible either. The astigmatism
correction prisms must be oriented correctly to the eye. >>
We do not grind in prism to correct astigmatism. We grind additional
curvature to the back of the lens at the correct axis in order to redirect
the light path. Prism is an entirely different "ball of wax" and is used to
change the optical center in eyes with muscle problems, eyes that are grossly
out of alignment, wandering eyes, etc. He was correct in saying that one
cannot grind axis correction into a dioptor lens, as turning the camera will
double the astigmatism. However, correcting the astigmatism in your eyewear
is the best correction and therefore will not turn when you turn the camera
vertically.
It is not a matter of which focusing screen will enable you to focus more
accurately...this is simply a matter of personal preference...it is a matter
of what your own, personal set of peepers needs in order to focus accurately.
Should you have less than 1.50 or 1.00 of astigmatism, you may be able to get
by with a plain ole spherical lens (regular) in your eyecup or use the
dioptor correction in your OM4T. Should you have more than 1.00 dioptor of
astigmatism, be prepared to perhaps have better focusing using your
prescriptive eyewear which has a specially ground astigmatic lens (where the
prescription has been ground to the specific axis which your poor ole
misshapen eyeball demands in order to focus at the correct spot in the back
of your eyeball).
This is not an easy topic, which is why people go to college and train for
years to understand. However, the above description might help those of you
whose eyeballs are eliptical (like a football) instead of round. We grind
your lenses to re-direct the path of light to the back of the eyeball....for,
if we did not, the oblong shape of your eyeball would make the point of focus
fall either short of the back of the eyeball, or beyond, and in either case
you could not focus properly. The reason you may "get by" with light
astigmatism, is that you can squint and get a temporary muscle contraction to
help out,but after the astigmatism gets to a certain strength, no amount of
squinting will help.
So, everyone, get out your prescription, read the second number in the
prescription, and if it is below a 1.50 dioptor, be happy. If not, get your
eyes examined every two years and wear them when focusing.
Susan Steele
Virginia USA
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