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Possible BA8 eyepieces and AFOV discussion

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#1 gmazza

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Posted 16 December 2012 - 05:44 AM

The possibility of the eyepieces of the BA8 binoculars series was speculated into topics by my research, but I stumbled on the two that very likely fit the high and low magnifications of each aperture.

The eyepiece series could be found here:

http://www.united-op...eces/EF_Eyep...

These are sold by several brands like BST, Astro Tech, Orion...

The 85mm BA8 have f/4.7 objectives

85*4.7=400mm (rounded focal length from 399.5mm)

The 19mm eyepiece features 65° AFOV, 5 elements in 3 groups

400/19=21.05, that's near the stated magnification for 22x85

The 27mm eyepiece features 53° AFOV, 5 elements in 4 groups

400/27=14.81, that's near the stated magnification for 15x85

The 70mm BA8 features 70mm f/4 objectives

70*4=280mm

280/27=10.37, that's near the stated magnification for 10.5x70

280/19=14.73, that's near the stated magnification for 15x70

The 50mm BA8 features 50mm f/3.7 objectives

50mm*3.7=185mm

185/19=9.73, that's near the stated magnification for 10x50

185/27=6.85, that's near the stated magnification for 7x50

This math leave me to consider the 19mm is the eyepiece on the high magnification binoculars of the series and the 27mm is the eyepiece of the low magnification ones.

User experience considerations, I only have the 10x50 and the 15x85 of this series, own these two for more than 3 years, was able to test the two eyepieces on these binoculars, the 10x50 shows considerably more field curvature than the 15x85. Even the binoculars and eyepieces being from the same pedigree, try to correct a f/3.7 objective with 65° AFOV eyepieces demands much more correction than to correct a f/4.7 objective with a 53° AFOV eyepiece.

Even with AFOV and field curvature differences my user experience with these two binoculars don't differ much, why ?

Discovered this only when I started to became more strict about IPD positioning, the merged field of vision is not that different in amplitude

Image in the 10x50 still looks flat and corrected enough in a binocular vision setting, the field curvature is only noted when seeing it with one eye, monocular like.

On the other side I don't experienced the 53° AFOV of the 15x85 binocular being restrictive, again, look restrictive when used just one side. When used as binocular does not result in a telescope like view.

In short, the binocular merged field put the image of these two in the same league, the 10x50 looks more expansive but the borders are not completely sharp, the 15x85 is more flat to the edge but the image lacks a bit on the lateral side the only downside in the 53° AFOV model is target acquisition (of course excluding obvious magnification differences).

Cost considerations, these eyepieces sell for U\$ 64 to U\$ 110, brand differences, and I wonder if there is a quality difference with prices. It's intuitive to think the "in factory" cost to add these for the binoculars would be less. Still the cost of eyepieces correspond to a larger fraction of the cost in the 50mm binocular than in the larger aperture counterpart.

In conclusion I could not positively affirm these eyepieces to be the ones in the BA8 binoculars, but for specs and math they are very likely and possibly will keep them on my wish basket to test in slower scopes.

#2 GlennLeDrew

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Posted 16 December 2012 - 03:30 PM

How did you determine the objective f/ratios? Does United Optics provide this information?

The eyepieces made for telescope use may not necessarily be the same as used in their binoculars. To begin with, the much shorter focal length of bino objectives results in a much more strongly curved focal surface. I would *assume* that for a higher end model of a maker's bino offerings, an eyepiece might be designed to better handle this strong field curvature. Of course, it might be a false hope.

In any event, it stands to reason that a minimal number of eyepieces would be made for a line of bino models, and I would not be surprised to learn that only two are used across the board.

#3 gmazza

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Posted 16 December 2012 - 04:33 PM

How did you determine the objective f/ratios? Does United Optics provide this information?

I got the f/ratios on Garrett website, tech specs

The eyepieces made for telescope use may not necessarily be the same as used in their binoculars. To begin with, the much shorter focal length of bino objectives results in a much more strongly curved focal surface. I would *assume* that for a higher end model of a maker's bino offerings, an eyepiece might be designed to better handle this strong field curvature. Of course, it might be a false hope.

Yes my thinking is in the speculative arena, but these eyepieces look to fit so well the binocular specs that I couldn't resist to put the possibility in discussion

#4 ronharper

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Posted 16 December 2012 - 04:51 PM

Interesting detective work and impressions. I agree that the way I view too, 53 deg is no impediment, nor blur at the edge of 65 deg a bother, provided the truly sharp zone is about 20 deg.

You might measure the diameter of the eye lenses, using the 1.25" barrel to set the scale, and compare that with your binoculars, as an added test of your theory. If there are believable eye relief specs, those could also be compared.
Ron

#5 GlennLeDrew

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Posted 17 December 2012 - 04:53 AM

If one has an eyepiece and bino in hand, a side by side comparison of reflections from the various lenses within each might indicate if they're of possibly identical design. While decidedly simple, this is a pretty potent 'pass/fail' kind of test.

#6 ronharper

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Posted 17 December 2012 - 11:59 AM

Glenn,
This is sort of tangential to this discussion but not completely.

Sometimes there are mentions of the "spherical like" and color aberrations caused by the thick prisms used in binoculars. Some wise people have said that binocular objectives and/or eyepieces must be made to counteract these prism effects, or performance will be substandard.

Anecdotaly, there is evidence that this may or not be true.
Some ATM types have reported dissappointing performance when prisms are removed from binoculars, and a straight-through finder scope is built from the objective and eyepiece. Henry Link, however, has reported that the eyepiece for Fujinon 16x70 performed admirably in an astronomical telescope.

What's your take on this? Could the BA8s get away with an eyepiece optimized for a perfect telescope?
Ron

#7 Andresin150

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Posted 17 December 2012 - 12:41 PM

I imported some of those BA8 binoculars directly from the factory (KUO) in 2010.
IIRC, they said that this particular series shared the same EPS, prisms and upper body for all apertures, that they just changed the objectives and their housing.
I only imported their higher magnification models (10x50, 15x70, 22x85 and 28x110) They claimed that all of them had the same eps, at 18.5mm
Considering that the focal ratios vary a lot (3.7, 4, 4.7 and 4.7 respectively) is understandable that some are optically different than the others (not just the same optics scaled, not a specific design for each as it should be); from my samples I concluded that the "better" ones where the 10x50 and the 22x85.
Strangely, the 22x85, which I liked the most and clearly outperformed every cheap 25x100 by far, was the most difficult model to sell, customers just wanted the "portable" units, or got aperture fever and went for the 110's...

#8 Rich V.

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Posted 17 December 2012 - 02:17 PM

I find it interesting that my WO 22x70s, though they share the BA8 body like the 22x85, apparently use a 20mm eyepiece rather than the 18.5mm used in the above mentioned binos. I had always assumed they used the same eyepieces as the others but the math says no.

Rich

#9 Andresin150

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Posted 17 December 2012 - 04:00 PM

Rich, the WOs are very different, apparently designed for them specially so they didn't sell that model to other vendors, (or probably because it is made by other company but are very much alike?)
At first glance the length of the body is longer than the common 70mm, so I can assume it uses longer focal length 70mm objectives (with ED glass?) and probably they used the same eyepieces, who knows, but in that configuration yielded higher magnification?, maybe the WO's are F=5.8?

#10 GlennLeDrew

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Posted 17 December 2012 - 04:10 PM

Ron,
At the low magnifications involved, prism-induced aberrations seem to me to be of no import, even considering the steepness of the light cones. The aberrations introduced by the objectives (and eyepieces) themselves dominate.

Over the years I've mated bino eyepieces to scopes and scope eyepieces to binos. Results mainly depend on the relative field curvatures, and how much of the objective is masked for the outer field.

One astonishingly decent combo was the Meade Research Grade 20mm Erfle and a Japanese made 80mm bino (formerly 15X.) The resulting 14X and 65 degree AFoV was marvelous, with axial sharpness being certainly as good, and near-field-edge definition holding up better than many another bino having a smaller AFoV. The eyepiece and objective field curvatures seemed to differ by not much. The 24.5mm prism clear aperture allowed the use of this eyepiece's larger field stop without introducing *visible* vignetting (it was present, as it absolutely must be, but not so bad as to be apparent.)

Such a mating (which would necessitate some changes to the back end of the body) available from the factory would have been a most attractive offering on the market.

#11 Rich V.

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Posted 17 December 2012 - 04:23 PM

Rich, the WOs are very different, apparently designed for them specially so they didn't sell that model to other vendors, (or probably because it is made by other company but are very much alike?)
At first glance the length of the body is longer than the common 70mm, so I can assume it uses longer focal length 70mm objectives (with ED glass?) and probably they used the same eyepieces, who knows, but in that configuration yielded higher magnification?, maybe the WO's are F=5.8?

It's an f6.2 FPL-51 ED doublet with a FL of about 420mm. EdZ measured the actual magnification closer to 21x so that works out to a 20mm eyepiece. The standard 18.5mm would have made it ~23x. There must have been a reason why they decided to create a slightly different eyepiece; perhaps it was intended to better match that longer focal ratio...

Rich

#12 F.Meiresonne

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Posted 17 December 2012 - 05:00 PM

Strangely, the 22x85, which I liked the most and clearly outperformed every cheap 25x100 by far, was the most difficult model to sell, customers just wanted the "portable" units, or got aperture fever and went for the 110's...

Well not this customer. I like the 22X85 very, but the 110 mm always putted me off due to the bulk and weight...and the lesser FOV

#13 Andresin150

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Posted 17 December 2012 - 06:13 PM

Probably, for the price it is probable that they designed all of its components.... I have not seen trough one of them, but EDZ's review made them very interesting

#14 EdZ

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Posted 19 December 2012 - 09:00 PM

Here's what I measured

GO22x85 mag=21.3 (exit pupil = 3.94) therefore with f given at 4.7, then F=399 and epF= 399/21.3=18.8

WO22x70 mag=21.1 (exit pupil = 3.36) therefore with f given at 6.2, then F=434 and epF= 434/21.1=20.6
can't expalin the difference inmeasured power and exit pupil, and got way to many notes to look thru to find it

Ob15x70 mag=14.6 (exit pupil =4.75) therefore with f given at 4.0, then F=280 and epF= 280/14.6=19.2

Ob10.5x70 mag=9.9 (exit pupil =7.1) therefore with f given at 4.0 then F=280 and epF= 280/9.9=28.3

edz

#15 gmazza

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Posted 20 December 2012 - 08:22 PM

Thanks EdZ,

Today I measured the 15x85

GO15x85 mag=14.78 (exit pupil = 5.78mm) so epF=399/14.78=26.99

The method used to measure the pupil was to take the photo of the pupil, infinite focused, with a macro lens at 1:1 so the image of the pupil projected on sensor is exactly the life size, knowing the sensor size was able to measure the pupil in any image editing program with ruler tool. This avoid the "dance pupil" issue when try to measure with a micrometer and parallax error

I think Andres clarified the doubt about the prisms and eyepieces,

they said that this particular series shared the same EPS, prisms and upper body for all apertures, that they just changed the objectives and their housing.
higher magnification models (10x50, 15x70, 22x85 and 28x110) They claimed that all of them had the same eps, at 18.5mm

Not sure if the individual ones being sold for telescope use will have exactly the same optical design or if one element of the eyepieces for binocular is different for field curvature correction. Will keep these eyepieces on my wish list for very decent performance at low price.

#16 GlennLeDrew

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Posted 20 December 2012 - 10:08 PM

Have you verified the 1:1 macro ratio by taking an image of a finely ruled ruler, or some object of accurately known size? Reporting a measurement from an image to the 10 micron level of precision implies high confidence, which should be justified.

#17 gmazza

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Posted 21 December 2012 - 04:46 AM

Have you verified the 1:1 macro ratio by taking an image of a finely ruled ruler, or some object of accurately known size? Reporting a measurement from an image to the 10 micron level of precision implies high confidence, which should be justified.

Yes I tested in past the precision of the macro lenses, the photo used was taken with Canon MP-E 65mm @ 1x

About decimal point confidence, there were small changes in size depending on the point of focus inside the eyepiece, so I choose the smaller point, that was inside the optical elements.

Small changes could happen, depending on the point of focus and the way the ruler tool was used to measure in software, I assume a margin of 0.2mm of error.

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