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Mark9473
Post Laureate
Reged: 07/21/05
Posts: 3216
Loc: 51°N 4°E
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Quote:
Suppose two binoculars have identical TFov, magnification and exit pupil, but one has a larger AFov due to more distortion. Since the amount of light leaving the exit pupil is the same, the one with larger AFov has a slightly dimmer appearing field at the periphery because the light has been slightly "diluted" by the distortion.
My Orion Mini Giant 15x63 and Docter Nobilem 15x60 both have a 4.1° TFOV (verified on the sky). The Orion has a large geometrical distortion at the edges resulting in it having a noticeably smaller AFOV, yet its image is also noticeably dimmer, compared to the Nobilem.
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Mark
Leica 8x20; Vixen 8x42; Swift 8.5x44, 10x50 and 20x80; TS 7x50; Orion 15x63; Docter 15x60
WO Megrez II 80 FD + Baader 90° T2 Amici
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EdZ
Professor EdZ
Reged: 02/15/02
Posts: 14732
Loc: Cumberland, R I , USA42N71.4W
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Quote:
2) To more fairly treat binoculars which have good illumination where it really counts; in the field center. For example, an instrument which delivers good central illumination can have its 'score' unrealistically lowered because of poorer edge-of-field illumination. (Recall that even a fairly narrow annulus at the edge of a circle--the field of view here--occupies a not inconsiderable fraction of the circle's area.) A quite narrow but more rapid fall-off in field brightness at the edge can have inflated consequences out of proportion to the real injury done.
Quote:
Glenn,
Nice post. I agree that illumination at the center is where it really counts. This is part of the reason I analyzed the data--it just didn't make sense that some of the binoculars with 5mm exit pupils were getting almost the same amount of brightness as the 7mm binoculars. I think we can now confidently say that the AFov is affecting the results just as you and Milt first pointed out.
-Dave
Keep in mind gentlemen, I measure for the width of central area that is 100% illuminated by another test and therefore binoculars that have a wider central area of full illumination score higher in that test.
This was exactly the basis of reasoning that influenced this test with the light meter. I had been measuring for years (with a target laser) that some binoculars had a much wider central area of 100% illumination. I suspected that would have an overall affect on the total illumination and that perhaps, all else equal, one binocular with a 40% diameter central area of full illumination (the Fujinon 10x50) should have a far greater total illumination than another binocular with only a 10% diameter central area of full illuimination.
See the 22x85,22x70,22x60 thread for this discussion in detail
I've stated before (and we discussed it in that thread) that the total illumination is the sum of the integrated illumination from the central area at 100% plus the rings each at progressively lesser illumination all the way out to the edge.
for example, total illumination is the sum of the
area of 40% central diameter x 100% illumination
area of the ring from 50% to 60% diameter x 90% illumination
area of the ring from 60% to 70% diameter x 80% illumination
area of the ring from 70% to 80% diameter x 70% illumination
area of the ring from 80% to 90% diameter x 60% illumination
area of the ring from 90% to 100% diameter x 50% illumination
essentially this is manually integrating the illumination. You could do this by calculus if you knew how, but I forgot how to do that 30 years ago.
Problem with this method is that it gives a result which is the percent of maximum potential illumination for that one binocular only. It doesn't compare one binocular to another.
So, I'll repeat, diameter of central area of 100% illumination is already accounted for elsewhere. I measure that with my target laser test, a test in which I map the illumination. And in fact, full illum % is listed above in my tables as one of my suspected variables. I have always suspected it as one of the most influential variables. But so far I have failed to prove it.
edz
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Teach a kid something today. The feeling you'll get is one of life's greatest rewards.
member#21
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Tony Flanders
Post Laureate
Reged: 05/18/06
Posts: 3469
Loc: Cambridge, MA, USA
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Quote:
I'm rather more inspired now to make a spot meter of sorts. I have an original model Sky Quality Meter (SQM). It has a sensor about 1mm square which has a tiny hemispherical lens attached directly. I've already done one quick 'n dirty experiment by kluging up a 'conical' attachment having an approximately 2mm diameter hole at the narrow end, yielding an FOV of perhaps 2 degrees.
Glenn, if you're measuring the light from the exit pupil directly, you shouldn't need a mask over the SQM. All you need to do is make sure that the illuminating source is hidden, and the binocular's eyepiece itself will be sufficient mask -- as long as you keep the SQM far enough away from the EP. Which, given the extreme sensitivity of an SQM, should be easy.
But do be careful to keep the SQM pointed directly toward the EP. What between reflections off the SQM's uncoated window, the fact that the sensor has a built-in lens, and the fact that the sensor isn't placed with extreme care, the directional sensitivity of an SQM is extremely strange and hard to characterize. (I'm speaking from experience, of course.)
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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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DaveL
member
Reged: 09/30/09
Posts: 12
Loc: WI, USA
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Hi EdZ,
Do you have the illumination measurements at the field edge for all the binoculars above? I can try the integration (or even the summing you did which is probably good enough) and see if the formula that results is indeed the best predictor. Hopefully it will also eliminate the dependence on the roof prism variable.
-Dave
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DaveL
member
Reged: 09/30/09
Posts: 12
Loc: WI, USA
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Glenn, Sounds Promising!! 
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GlennLeDrew
Pooh-Bah
Reged: 06/18/08
Posts: 1296
Loc: Ottawa, Ontario, Canada
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Tony,
I know exactly what you mean regarding the SQM. I've had mine apart already in order to see how it's constructed. Your simplification of the technique in using the SQM 'as-is' does have some merit. I'll give it a try and see how easy (or difficult) it is to maintain distance and alignment w.r.t. the exit pupil.
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Home-made 11X50 right angle bino, 8.1 deg. FOV
Modified 26X100 bino, 3.5 deg. FOV
Home-made Mk II RA bino, using interchangeable objectives and eyepieces
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Mediocre minds discuss people. Good minds discuss events. Great minds discuss ideas.
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