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jkevn
sage
Reged: 03/02/06
Posts: 211
Loc: Tucson, Arizona
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I was wondering how subjective the evaluation of off-axis performance is in different binoculars, mostly curvature of field, depending on the person's age and degree of prebyopia. I think that a young person may be able to accomodate, to some extent, for a change in focus from center to the outer edges of the field of view. Not for astigmatism or coma, of course. but the eye automically compensates for a change in focus, if it can. After the age of 40, almost everyone begins to lose the ability to change their eye's focus. In my case, at age 55, I've had cataract surgery in both eyes to remove cataracts and implant acrylic lens replacements, and although I have now near 20/20 distance vision without glasses (a lot better than before), I have zero ability to change focus. So what I see might be a good measure of how flat the field really is...just a thought.
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Rich V.
Pooh-Bah
Reged: 01/02/05
Posts: 1393
Loc: Carson Valley, Nevada, USA
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Good point. You're the perfect field curvature "tester"!
I never really noticed FC when I was younger; now at 60 I see it easily. Maybe now I'm pickier than I was years ago but it's more likely my eyes are just not up to the job anymore.
Having a flat field is important for astronomy; the 70mm and 100mm bins I currently use have relatively little field curvature either because of field flattener eyepiece elements or longer focal ratio to minimize the problem.
During daytime viewing I don't notice whether the field is flat or not. 
Rich V
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Binoculars:
33/50/75/150x100 Saturn III, 16x70 FMT-SX, 10x50 PCF-V, 10x43 DCF-SP, 10x35 E2, 7x35 E, 8x30 E2, 7x26 Custom, 8x23AS Diplomat, 8x23 Travelite
Scopes:
C9.25, 6" f8 reflector, SV80S
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Tony Flanders
Post Laureate
Reged: 05/18/06
Posts: 3464
Loc: Cambridge, MA, USA
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Quote:
I was wondering how subjective the evaluation of off-axis performance is in different binoculars, mostly curvature of field, depending on the person's age and degree of prebyopia. I think that a young person may be able to accomodate, to some extent, for a change in focus from center to the outer edges of the field of view.
I'm absolutely sure that's true. And during daytime, when your pupils contract and lengthen the effective f/ratio, field curvature matters less.
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Tony Flanders
First and foremost observing love: naked eye.
Second, binoculars.
Last but not least, telescopes.
And I sometimes dabble with cameras.
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EdZ
Professor EdZ
Reged: 02/15/02
Posts: 14724
Loc: Cumberland, R I , USA42N71.4W
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Quote:
During daytime viewing I don't notice whether the field is flat or not.
In large part, that's because field curvature has the effect of bringing closer objects into focus in the outer edges of fov. So, for daylight viewing, field curvature may not be considered problematic at all, as it actually provides benefit to the viewer who wants to see the entire view in focus.
edz
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Teach a kid something today. The feeling you'll get is one of life's greatest rewards.
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