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Re: (meteorobs) Spectacles and LM
This has been quite a thread! But, I would like to at least
consolidate some ideas presented and merge with my own
experiences and basic optics.
1. There should be no great difference in performance between a
properly prescribed set of contacts and spectacles ON AXIS,
(assuming the wearer has not grossly aberred vision).
As long as the lens corrects all aperture rays to a near-perfect
point image, the LM will be the same. The difference comes if there
is a lot of astigmatism or other asymmetric aberration, because the
mass-produced contacts cannot be made to correct those as well
as the custom ground spectacle lens, which also is held in a fixed
orientation w.r.t. the eye.
2. Off axis the contacts are definitely superior!
Since they completely cover the iris FOV, and are very thin, there
is no difference on or off-axis correction, and your peripheral acuity
will be as good as the natural eye. Spectacles are quite poor off
axis. Not only is there a physical limit on the FOV due to lens
size, but their thickness and distance from the eye causes oblique
rays to be poorly corrected. Only near axis correction is optimized
by the optical shop. Further than about 20-30° off axis, the
correction degrades with various aberrations, including field
curvature.
3. Optical transmission will be similar.
The contacts are very thin compared to spectacle lenses, so the
absorption will be less. However the difference is small. I am not
sure of the relative transmissibility of the various lens compounds,
but the difference will be negligible, unless extreme thick spectacle
lenses are necessary.
4. Contacts don't fog or frost over like spectacles.
5. Contacts can cause physical discomfort problems, resulting in
tearing(water) and corneal irritation, which can seriously degrade
vision compared to glasses.
This problem is highly variable. Some people tolerate contacts
better than others and can wear them for long periods without need
for eyedrops. Also the ambient temperature and humidity affects it
a lot. Observing from a hot, dry location with a wind in your face will
rapidly dry them out and cause problems. Conversely, I find in a
cold place, with a light wind, the slight irritation and tearing
provides a natural wetting agent allowing long periods of
comfortable viewing. (I can view all night on top of Haleakala at 40F
but get into trouble quickly at 70F and need to add drops every few
minutes if there is a breeze in my face.)
5. Glasses, due to their weight, can become physically
uncomfortable sitting on your nose for hours on end. And if its very
cold, conduction through the frame will start you itching like crazy.
Conversely, properly fit and lubricated contacts are practically
undetectable.
6. Contacts are generally not permeable enough to O2 to allow
sufficient supply to the cornea (It gets its O2 essentially all from
contact with the ambient air)
So, this is the primary limitation to wearing contacts for extended
periods of time. The eye will get very irritated due to O2 starvation
and neo-vascularization sets in.
So, In summary I believe contacts take the edge over spectacles
for Meteor observing. The only conditions that spectacles hold
advantage is:
1. Warm, dry, windy locations.
2. Grossly aberred vision that cannot be sufficiently corrected with
contacts.
3. Extremely long observing sessions. (>12 hr?)
Thats all folks!
Mike.
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