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>>graduated ones with polarizing
magnetic clip-ons <<
Interesting notes Terry. I have tried a similar
setup but one of our club members warned us of a significant danger in graduated
lenses for gliding . .
In his considered opinion we should have a
driving/gliding prescription made to give 'exact focus at infinity with the eyes
at rest' to see those vital threatening specs during lookout, not to mention
dust and birds that mark the thermals! A
graduated lens apparently does not do this well as you cannot rely on
your eyes always being focussed at infinity.
Now, having the 'focus at infinity with
the eyes at rest' means that our short vision is poorer than normal - being
exacerbated by the main lens being set longer than for normal civilian use.
This means that a (narrow) bi-focal region is usually needed -
focussed to see maps and instruments at arms length (perhaps 300 to
800mm?).
Perhaps I can try to explain further. If the opto
does his normal job he will choose a lens that works fine at (guessing the
numbers here) 20m and infinity, but is actually best at 200m and 'pretty good'
at 20m and infinity. David suggests that the sweet spot should be at
infinity, not 200m. Thus the lens might work 'pretty well' from say 50m to
beyond (!) infinity but is actually perfect at infinity, thus making a
greater need for bi-focal or even trifocals. My first set made like this have
been great although the bifocal part is too wide and next time I'll get it made
to '28' not '35'. Looking forward to the benefits of polarizing
too.
I've also been contacted offline by a fellow
pilot been using such a setup for years - no worries.
>>It would be interesting to know if it's possible to get
"clear", that is non-tinted polarizing glasses (or clip-ons) as used in
photographic polarizing filters<<
What a great idea Terry!
This has been a most interesting thread, I hope
that we all get to benefit in the air! Thanks to all for your
valuable contributions.
Cheers,
Jim Kelly
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