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Not all prisms are created equal

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#1 GlennLeDrew

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Posted 13 October 2009 - 05:49 PM

Until now I'd been using a less-than-optimal laser line for my bino prism testing; its 20 degree fan angle is too wide and results in a fainter-than-desirable line. Last night I made a 'custom' line generator lens for my laser pointer. It has a fan angle of 5 degrees, and moreover has a more even light distribution.

I tried it out on a couple of bino bodies, one of which is the 'junker' Bushnell Sportview 8X40 I've already reported on. The brighter laser line certainly makes things easier to see!

To recap the method of the test... The laser is taped to a wall, its line oriented and aimed horizontally. At a distance of about 8 feet, I place the bino body (objectives and eyepieces removed) so that the prism apertures are intercepting the line. I then send the reflections back in the general direction of the laser. When both prism assemblies are intercepting the line, two groups of 4 short line segments are seen, there being 4 for each because a Porro has 4 air-glass interfaces.

Well, as I was positioning and rotating the bino body, I noticed a couple of unexpected effects:

1) At all times every line segment was brighter at each end point. That is, the lines were of even brightness except at the *very* ends.
2) When the line was slicing the aperture well off-center and hence making a shorter chordal cut, the projected line segments became astonishingly *non-straight*. The form of line curvature was such that when the laser line cut across the upper edges of the prism apertures, the lines took on a 'smiley face' aspect, and when cutting across the bottoms the lines were 'sad.'

And I'm talking about some severe curvature here, whereby some curled a full 180 degrees! Both effects are related, and indicate significant turned edge. This could be further exacerbated by the fact that a couple of the apertures have segments of the prism edges intruding a small distance into the optical path. This latter problem results from the combination of using prisms *just* barely large enough to begin with, and having to position them somewhat off center in order to achieve collimation.


After this surprising result, I whipped out my 8X25 monocular in order to check the visual result of these really bad turned edges. For this test, I simply hold the monocular close to and in line with a prism aperture, and sight a distant target, faraway street lights in this case. I wasn't surprised here!

The other bino body I was examining as well, a Tasco 'World Class' 7X35 extra wide angle, held up *very* well. (Under the laser line test, at all times the projected lines remained completely straight.)

The Bushnell, however, was absolutely dismal. In addition to significant spherical aberration, there were all kinds of flares and spikes projecting from the target lights. Just an optical mess.


So there you have it. There are *many* areas in which a manufacturer can and will skimp on when cobbling up those lower-end lines of binos.

#2 werewolf6977

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Posted 13 October 2009 - 08:59 PM

Thanks for the info.

#3 Bensi

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Posted 14 October 2009 - 02:04 AM

Interesting analysis and testing.
The translated with Google because I can not understand everything ... but what I realized was enough to get you compliments.
Thanks

#4 Mateyhv

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Posted 14 October 2009 - 03:22 AM

Hi Glenn, could you post some pics? Thanks

Matey

#5 KennyJ

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Posted 14 October 2009 - 10:46 AM

Not altogether surprising , but very interesting , Glenn .

For instructive purposes , these are the kinds of experiments that would lend themselves very much to video recordings .

It would be far easier to follow , understand and memorise in detail if seen that way .

Kenny

#6 Rich V.

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Posted 14 October 2009 - 12:21 PM

Glenn, your test shows how important all the optical components of a binocular are as a whole. I can see how well figured objective and eyepiece elements can easily be degraded by a poorly ground/aligned prism assy. Binocular design and production is complicated and there are a lot of places where things can go wrong.

Gordon has pointed out more than once that the aberrations of each prism surface are compounded. I've wondered lately whether the messy high mag. star images (i.e. @150x compared to small refractor scope) of my Miyauchi binoculars are less a product of objective figure plus CA and more a product of the rather complex 45° Schmidt roof prisms plus the rhomboid turret prisms in the optical path. That's a lot of prism surfaces compared to none! I know it's certainly a combination of all aberrations but maybe the prisms deserve more of the blame. What do you think?

Thanks for posting these interesting experiments; they are always thought provoking!

Rich V

#7 Simon S

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Posted 14 October 2009 - 12:45 PM

Interesting what you say about Tasco World class binoculars, as my pair are optically poor and almost unusable.

#8 GlennLeDrew

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Posted 14 October 2009 - 04:52 PM

The Tasco 'World Class', model 435, seems to have rather decent prisms. And I've found that the objectives perform quite well, too, mainly because they don't have significant turned edge. But the eyepieces are what they are; an extra wide angle design which is forced to work with f/3.6 objectives. However, on axis the images are fine, and so I'd judge this bino overall to be a decent enough instrument.

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I do plan to take photos of the setup and resulting patterns of projected lines. This has to be done in a relatively darkened room, and so I'll first have to obtain an extra tripod in order to steadily hold both bino body and camera. (Unless I kluge up a 'bracket' which attaches to the same tripod the bino is mounted on. Hmmmm.)

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Prisms act like *very* weak negative lenses, even if all surfaces are perfectly flat. I've seen this amply demonstrated when peering through any prism assembly with my 8X25 monocular. If I first focus on infinity, when aiming through the prisms I then have to focus a little closer than infinity, which is the effect a negative lens would impart.

However, this degree of effective curvature is so slight that I can't imagine spherical aberration becoming significant even at magnifications on the moderate-to-high side, if the prisms are well made, of course.

For compact binos which employ short focus objectives, it's concievable that chromatic aberration could become apparent at higher powers, but perhaps only if the other optical elements supress secondary spectrum sufficiently well. Indeed, the instrument might have to qualify for the appellation, "apochromatic" where at least the objectives are concerned.

If you get the impression I'm doubtful that prism-induced aberrations in a well made bino which works at lower--or possibly moderate--magnifications should contribute in any significant way, then you'd be right. Even if the objectives are spherically corrected apochromats, eyepieces typically introduce their own aberrations which should quite effectively mask prism aberrations.

At least, that's the impression I'm increasingly leaning towards.

#9 milt

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Posted 14 October 2009 - 06:32 PM

I've wondered lately whether the messy high mag. star images (i.e. @150x compared to small refractor scope) of my Miyauchi binoculars are less a product of objective figure plus CA and more a product of the rather complex 45° Schmidt roof prisms plus the rhomboid turret prisms

Hi Rich,

Exactly how are your 150x star images "messy"? If you use a monochromatic filter do they improve? The Saturn III's f/7.5 objectives should be considerably kinder to its prisms than most binoculars.

Thanks,
Milt

#10 Rich V.

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Posted 14 October 2009 - 08:44 PM

Milt, what I mean by "messy" is that brighter stars focus to a round blob similar to the way stars look in a SCT during poor seeing. Stars won't focus as tight as a refractor scope, for instance.

The purple blur of CA is obviously softening the star image as well. At 75x or less, a range more in line with an achro binocular, it isn't nearly as bothersome. Your idea of trying a filter is a good one and I'll have to give it a go when it clears.

I've never star tested a similar sized achro refractor for a comparison with the Miyauchis but I'm thinking the scope with only one mirror reflection would put up a way better star test than a prismatic binocular. I suppose Miyauchi's making a 150x eyepiece set for the Saturns was an invitation to show off their binocular's shortcomings!

I never expected a binocular at 150x to throw up a perfect disk/ 1st diffraction ring pattern like my 80mm apo refractor does. It's more that I'm just wondering what the sources of this imperfect star image are and I was suspecting that the five prism reflections may be a good part of the problem. Perhaps it's simply that the objectives are just not figured for "planetary performance" and the prisms have nothing at all to do with it.

It would be really interesting to push your Kowa to 150x somehow and see how it star tests. I know it has a better figured and corrected objective; I wonder how well the prism assys. transmit that image to the eye. How good can a binocular be at 150x or even 75x for that matter? We may never know.............

Rich V

#11 milt

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Posted 15 October 2009 - 07:33 PM

I'm just wondering what the sources of this imperfect star image are and I was suspecting that the five prism reflections may be a good part of the problem. Perhaps it's simply that the objectives are just not figured for "planetary performance" and the prisms have nothing at all to do with it.

It would be really interesting to push your Kowa to 150x somehow and see how it star tests. I know it has a better figured and corrected objective; I wonder how well the prism assys. transmit that image to the eye. How good can a binocular be at 150x or even 75x for that matter?

Rich, I agree that's asking a lot of any binocular - well into the binoviewer arena. Heck, premium Amici prisms aren't even recommended for use much over 100x. A couple of comments on the Highlander:

When I was working on my review, I contacted Kowa's designer (also an avid astronomer - probably the only reason the Highlander even exists) through Kowa Optimed. I was told that the prisms and objectives were designed to work as a system and were optimized for center-of-field. That tells me that the objectives themselves might not be as good without the prisms and vice-versa.

As for magnification, while I have never used mine over 64x, Markus Ludes reported modifying 5mm Naglers for his to reach 90x - but that's still way below your 150x. I would be surprised if they gave a decent star image at that power. So I'm suggesting we both continue to use our fine binoculars at more sane magnifications. :waytogo:

Best,
Milt

#12 orlandog

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Posted 15 October 2009 - 08:39 PM

Hi,
To confirm Glenn’s original statements and Milt’s thoughts on practical bino magnification limits a bino fabricated from Mead objectives (70mm D. - 600mm fl.), a vintage Tasco body with the original prisms and 9mm volcano top Ortho’s gave, this past month, crystal clear images with pinpoint moons of Jupiter at 67X. – more than a pleasant surprise.
Some photos of this bino at:

http://picasaweb.goo.../hrrrose/Bino3#

I agree, Glenn the vintage Tasco prisms provide outstanding performance. In addition, Milt what is the point of stretching binoculars, as they are currently constructed, into the realm of telescopes/binoviewers.

#13 milt

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Posted 16 October 2009 - 03:27 PM

a bino fabricated from Mead objectives (70mm D. - 600mm fl.), a vintage Tasco body with the original prisms and 9mm volcano top Ortho’s gave, this past month, crystal clear images with pinpoint moons of Jupiter at 67X. – more than a pleasant surprise.
Some photos of this bino at:

http://picasaweb.goo.../hrrrose/Bino3#

Nice setup - thanks for sharing your pictures. I'm guessing the finder brackets are somewhat challenged to hold collimation at 67x!

As for why we push our binoculars into binoviewer country I can't speak for Rich, but in my case I always use the Highlander on its own rather than as an adjunct to a telescope. When I'm on an object that could use more than the standard 32x, I really like being able to double it. However I have no plans to mutilate any more Naglers to go higher... :bawling:

Sorry to derail your thread Glenn.

Milt

#14 GlennLeDrew

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Posted 06 November 2009 - 06:33 PM

A couple of nights ago I examined both the Bushnell and Tasco prisms via the Ronchi test. But in both cases I had to install a longer focal length objective. Why? Because the existing objectives (40mm and 35mm aperture) have the focus lying a bit *inside* the bodies. The Ronchi test is best done with the eye a little inside focus because it's easier to obtain full illumination of the aperture under test. Therefore a longer focal length 50mm objective was used so as to place the focus handily outside the bodies.

The test was done by simply pointing the bino toward a bright but fairly distant sodium street light, and looking throug the grating held at the back end. This test is not as 'complete' as the laser line test because the converging light cone results in smaller and smaller portions of the prisms being used farther back along the optical train.

In any event, the first aperture and the immediately following reflective surface were fairly well sampled. The Tasco looked great, with nice straight lines and the expected diffractive effects at the edge of the aperture. But the Bushnell was awful! While most of the central portion of the aperture produced acceptably straight Ronchi lines, the edge was ghastly, exhibiting what I'd estimate to be easily a couple of waves of turned edge.

#15 KennyJ

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Posted 06 November 2009 - 07:06 PM

I've occasionally pondered that if the once celebrated British musician Mike Oldfield ever decided to create a 40th anniversary sequel to his classic 1973 album " Tubular Bells " he could do far worse than name the new 21st century work " Turning Edge " .

It's not so much the turned , or indeed , UNturned edges aspect that intrigue me , but that pricelessly brief moment in history when edges can be actually caught in the presumably despicable act of turning .

Kenny

#16 DJB

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Posted 07 November 2009 - 02:14 AM

Love that album Kenny.


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