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Tony Flanders
Post Laureate
Reged: 05/18/06
Posts: 3461
Loc: Cambridge, MA, USA
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As threatened, I've been doing some experiments trying to characterize effective aperture toward the edge of the FOV using a slight modification of the method proposed by Glenn LeDrew.
Like him, I shine a light through the eyepiece and measure the light cylinder coming out of the objective. But instead of measuring it directly, I project it through a piece of 5-mm-ruled graph paper and then photograph it, so I can measure it at leisure.
When the light source is on the EP's optical axis, the result is a circle, or nearly so, for any respectable binoculars. But when the source if off-axis, you get vignetting from various parts of the bino, and the effective aperture is far from round.
It's hard to explain in words; look at the attached photo mosaic instead. I was measuring a pair of 1960s 7x35 Nikons and my Canon IS 10x30s -- the only binoculars I own that come close to their nominal aperture, BTW. Note the prism edges in some of the photos, especially for the 7x35s 15 degrees off-axis.
The more vignetted the aperture is, the dimmer the view will be through the eyepiece at the corresponding distance from the center of the field of view.
The aperture measurement can be pretty accurate this way, but I would need much fancier equipment to control the angle of the light source with real precision. For instance, the fact that the ovals in my photo are tilted reflects the fact that the light source wasn't centered accurately on the vertical axis. And the horizontal angles quoted could easily be off by as much as 5 degrees.
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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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ronharper
Pooh-Bah
Reged: 02/14/06
Posts: 1276
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Interesting Tony, thanks a lot for coming up with that. There is a very old fashioned drafting instrument called a planimeter that calculates the area of an arbitrarily shaped region as a wheel is rolled around the boundary.
One thing I notice is that the evil shady square, the sign of the evil BK-7 prism I suppose, only becomes evident off axis. I know I have seen it in some old binos with the exit pupil centered in the eyepiece. Looks like Nikon figured out how to make a good binocular, at least on axis, back in the old day. Ah, BK-7, Kellners, planimeters, the old ways are being lost, but they really did work once upon a time.
Ron
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EdZ
Professor EdZ
Reged: 02/15/02
Posts: 14721
Loc: Cumberland, R I , USA42N71.4W
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this test produces the same results as the target laser test.
Similar photos can be found thru the Best Of threads "Aberrations and Deficiencies" and "Measuring/Testing Optical Aspects"
Aberrations and Deficiencies - follow links to "vignetting" and "Illumination of the Exit Pupil" and "GO22x85s"
Measuring/Testing Optical Aspects - follow links to "EdZs GO22x85" for a pictoral essay showing obtaining these same results using a target laser.
the only differences being, I refer to this as Illumination of the Exit Pupil, and I measure the points on the objective that provide full illumination and then progressively smaller percents of full, out to the edge.
another way to skin the cat.
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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ronharper
Pooh-Bah
Reged: 02/14/06
Posts: 1276
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Ed,
Thanks to a recent discussion I finally realized what you had been doing for some time in your tests. I had been confused, just due to your choice of wording that I couldn't quite get hold of. I'm with you now, and thanks for all those efforts.
Just holding the bino up and watching the exit pupil deform as it moves off axis and approaches the field stop tells a lot, in a quickie qualitative way.
Ron
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Tony Flanders
Post Laureate
Reged: 05/18/06
Posts: 3461
Loc: Cambridge, MA, USA
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Quote:
There is a very old fashioned drafting instrument called a planimeter that calculates the area of an arbitrarily shaped region ...
Insofar as it's the intersection of two circles, which is true for the most part, this is also easy to calculate analytically.
Quote:
Looks like Nikon figured out how to make a good binocular, at least on axis, back in the old day.
Yes, that's the very first pair of non-toy binoculars I owned; my father did a lot of research when picking them as a present for me. And they're the ones I used when I was learning the night sky. I don't use them much anymore, but I keep them in the office for unexpected needs -- which do arise every month or two.
Minimal coatings and mediocre control of stray light, but great eye relief, wonderful ergonomics, and quite sharp.
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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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GlennLeDrew
Pooh-Bah
Reged: 06/18/08
Posts: 1250
Loc: Ottawa, Ontario, Canada
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Tony,
Nice job! Those pictures certainly say lots.
Note how the internal restrictor in the 7X35 is blurred, while that in the 10X30 is sharp. To appear so fuzzy suggests that the aperture is quite close to the focal surface, and I suspect it's the field stop edge, it being not quite at the focus. This could be verified by racking the eyepiece in/out.
That's another thing I've observed with a number of binos. The field stop is part of the tube the eyepiece slips over, and is not part of the eyepiece itself. This means that as seen through the eyepiece, the field stop will appear nicely sharp for just one position of focus. As you can imagine, I prefer field stops to be built into the eyepiece assembly because they'll always appear as sharp (if correctly positioned during manufacture, of course).
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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
My Gallery
Mediocre minds discuss people. Good minds discuss events. Great minds discuss ideas.
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Tony Flanders
Post Laureate
Reged: 05/18/06
Posts: 3461
Loc: Cambridge, MA, USA
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Quote:
Note how the internal restrictor in the 7X35 is blurred, while that in the 10X30 is sharp. To appear so fuzzy suggests that the aperture is quite close to the focal surface, and I suspect it's the field stop edge ...
No, it's not the field stop -- I checked that.
I'm 99% sure that I do know what it is, though: the inner edge of the piece of plastic that holds the eye lens in place. You can see it easily looking down the objectives. The reason that it looks fuzzy is that it *is* fuzzy (not made of metal).
Doing the math, I realize that in order to avoid vignetting, the minimum possible size for the eye lens is roughly:
EXIT + 2 * tan(AFOV/2) * ER, where
EXIT is the width of the exit pupil
AFOV is the apparent field of view
ER is the eye relief
I can't measure the eye relief precisely with the tools I have at hand right now, but it's probably on the order of 15 mm. Exit pupil is 5 mm and AFOV is 50 degrees. That requires at least a 19-mm eye lens, and the portion of the eye lens that isn't covered by the holder is about 16 mm across.
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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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milt
professor emeritus
Reged: 09/13/04
Posts: 551
Loc: Arizona
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Quote:
the horizontal angles quoted could easily be off by as much as 5 degrees
Tony, this isn't as bad as it seems. Again, the benefit of working at the exit pupil is that the sensitivity to errors is reduced by the magnification. So a 5 degree error would translate to 0.7° at the objective for the 7x and 0.5° for the 10x. When inserting a laser beam into the objective at an angle I found it very difficult to achieve any better accuracy.
Milt
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Philip Levine
super member
Reged: 03/22/07
Posts: 156
Loc: near Boston, MA
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Hi Ron,
I wondered if I would ever find a use for a planimeter that came into my possession many years ago. Just happened to try measuring some binocular aperatures recently. Using the quick measurement technique I did notice if the flashlight is not positioned just so, the projected aperature "circle" is not always round.
For those who want to see what a planimeter looks like, please see the attached photo.
Phil
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Zhumell 10" Dob (modified with degree circle and Wixey inclinometer)
26mm 2" GSO eyepiece
9mm 1.25" GSO eyepiece
13mm Nagler T6
Crayford focuser
DewBuster w/homemade heater strips
Nikon 10x50 EX binoculars
University Optics 11x80 binoculars
Fujinon 16x70 FMT SX
Surveyor Tripod w Universal Astronomics Parallelogram
member International Dark-Sky Association
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ronharper
Pooh-Bah
Reged: 02/14/06
Posts: 1276
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Phil,
That is a lovely and impressive contraption. No irregular region is safe.
Ron
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Luigi
Post Laureate
Reged: 07/03/07
Posts: 4933
Loc: MA
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Another traditional method of area calculation is to trace the curve, cut it out, and weight it on an accurate scale. You can weigh a rectangular piece of paper to get it's weight/area and use that to get the area of an arbitrary shape from its weight. With a good scale and good paper, you can do substantially better than 1% accuracy. For these purposes, a powder scale used in reloading ammunition would work well enough.
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17.5" f/5 Dob. IM-715 MCT. 120ED. Lunt 60mm Ha.
Zeiss, Leica, Fujinon, Nikon, Pentax, Bushnell bins
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