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Kowa BD II 10x42 XD Optical Tests (1 Viewer)

henry link

Well-known member
I borrowed a friend’s newly purchased Kowa BD II 10x42 XD and ran some of my usual tests. Here are the results.

FOV, AFOV

For comparison I used a Nikon 10x35 EII with a FOV I’ve measured to be 7º at infinity focus. The FOV of the Kowa 10x42 was very slightly smaller, perhaps around 6.95º, a little more than 0.2º below Kowa’s spec of 7.2º.

The AFOV measured only 64º. That was considerably smaller than Kowa’s spec of 72º. I expected it to be off since Kowa’s spec used the simple formula of multiplying the magnification by the real field which always exaggerates the true AFOV. In this case the real field spec was also exaggerated causing the calculation to be even more off. There was one other contributing factor to the low AFOV, a particular distortion profile which causes the true AFOV to be smaller than expected (see the next section).

DISTORTION

The two photos below on the left illustrate distortion in different ways. The far left photo was made be placing the eyepiece of the binocular about 2 feet from a set of mini blinds and aiming the camera through the objective lens. The straight blinds appear wavy because of a compound distortion sometimes referred to as “mustache” distortion because it causes straight lines to curve into handle bar shapes in the outer part of the field .

Because the photo was made through the objective end it shows the distortion reversed from the way it appears when viewed through the eyepiece. In the photo barrel distortion appears in the center half of the field reversing to pincushion distortion at the field edge. In normal viewing the pincushion is in the center and the barrel is at the edge. Some expensive binoculars use this same distortion profile, like the Swarovski SV and the Zeiss SF. The Kowa profile looks quite similar to the Zeiss SF.
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The second photo demonstrates how this compound form of rectilinear distortion gives rise to a panning disturbance called the “Globe Effect” or ‘Rolling Ball” The row of six circles was photographed through the Kowa with the field edge on the right. The circles extend from the field edge to about half way to the field center on the left. Notice that there is a gradual horizontal compression of the circular shape from left to right until the last circles near the field edge seem to be foreshortened as if they are moving around the edge of a rotating globe or ball. Another result of this distortion is a smaller than expected AFOV as objects near the edge are crammed into a compressed space.

A few other things are also visible in that photo. Notice that some of the circles are better focused at the 12:00 and 6:00 positions than at 9:00 and 3:00 indicating astigmatism. While this target doesn’t show lateral chromatic aberration very well, it is visible in the form of blue and orange fringes. The last circle on the right has less astigmatism and lateral color because it’s vignetted by somewhat undersized prisms which cause the soft shadow extending into the field from the field stop.

CHROMATIC ABERRATIONS

Longitudinal CA is a non issue, but there is considerable lateral CA, even quite close to the field center. Lateral CA seems to be one of the aberrations for which it’s most difficult for different observers to reach consensus. Even poor performing binoculars won’t show it very vividly under most lighting conditions and it can be artificially induced in good performers through incorrect pupil positioning. I’ve long used a high contrast black and white target designed to induce some amount of lateral color in every binocular. Only by using that target in consistent repeatable conditions and having reference standard binoculars for comparison can I sort out the wheat from the chaff. For this test I used a Nikon 10x35 EII as a reference. The Kowa showed much more lateral color than the Nikon on my target particularly in the most important area near the field center. It’s not possible to say whether the lateral color is generated by the eyepiece or objective group or both, but I’m beginning to suspect that the recent crop of short, light binoculars is pushing past the limits of how low focal ratios can go without performance compromises in this area, even with ED glass.

GLARE

The photo of the binocular interior below shows the causes of the Kowa’s less than perfect glare resistance. The light source is an overcast sky with the binocular pointing unto a dark area in front of it to simulate a field condition that often causes veiling glare; a dark landscape under a brighter, but not necessarily very bright sky. The bright reflections at the bottom edge of the dark exit pupil come from both the objective lens retaining ring and the edges of slightly undersized prisms. When I look through the binocular I see a FOV that's dark toward the top, but flooded with a bright gray haze of veiling glare across the bottom half. That’s the bright reflection at the exit pupil bottom edge as seen far out of focus by the eye.

This isn’t the worst I’ve seen. Even some respected favorites like the Swarovski 8x30 Habicht and 8x32SV are worse, but all these small exit pupil binoculars with reflections near the exit pupil have one thing in common: they tend to show their veiling glare to the eye more readily than larger exit pupil binoculars simply because the exit pupil edge is more likely to enter the eye’s pupil under the lighting conditions that produce the glare.

RESOLUTION

I used a USAF 1951 glass slide target at 10 m with the magnification boosted to 80x. I only measured the full aperture resolution.

The left side was 5.6’ (235/D), the right side 6.5” (270/D). These numbers are bad and worse. For comparison I measured one side of the 10x35 EII at 4.00” (140/D)

STAR-TEST

As expected, a very poor star test brought some explanation for the poor resolution. So much was wrong that the star-test was not very easy to read, but I could see poor correction of spherical aberration, coma from misaligned optics, astigmatism and very defective roof prisms with exactly the same appearance in both sides and which seemed to be aligned with the astigmatism.

The roof prisms really require their own analysis. I’ve never seen anything close in another binocular. The roof edge appears to be so blunt that it is clearly visible as a dark line in the photo below of an extremely defocused star point. Besides completely screwing the star test even if nothing else were wrong this roof edge shows up as fairly extreme spikes perpendicular to the line of the roof edge radiating from any small bright light source in a dark field like a street light. I noticed that Allbinos mentioned the spikes in their overview. In the photo you can also see the edge of an undersized prism cutting into the exit pupil on the left side.

I would be interested to know if others see this same thing in their Kowa’s. To see and photograph the defocused artificial star I focused the binocular at infinity and examined the artificial star at about 3 meters distance.

CONCLUSIONS

This is a binocular I don’t plan to take a second look at, but I couldn’t help but notice that even with all it’s shortcomings I don’t think it’s likely to look obviously bad to a casual hand holding observer. That’s a testimony to just how much can be gotten away with in the world of binocular optics. The image through a low power, hand held instrument can look relatively OK, even when its optics are objectively pretty awful. I could largely dispel that illusion by mounting the Kowa on a tripod next to a Nikon 10x35 EII and focusing on a resolution chart at 30 meters. Then there was quite an obvious difference between the sharp line pairs viewed through Nikon (not itself the sharpest binocular in the world) compared to visibly softer ones viewed through the Kowa. I suspect the flaws of the 10x42 are in the other models as well, possibly even more visible in the 10x32, but probably less visible the 8x42 and especially the 6.5x32 where the flaws will be more forgiven by lower magnification and in daylight benefit from the lower aberrations that result from larger exit pupils.

Henry Link
 

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Binastro

Well-known member
Thank you for the very fine test.

One barrel of my sample of the VisionKing 5x25 is considerably worse than the Kowa 10x42 you tested.

Is the use of molded optical elements one of the reasons for the bad performance of some Chinese made binoculars?

Regards,
B.
 

dries1

Member
Henry,

Thanks for taking the time to do this. I do not see the same view as your third photo from the left in the 8X42. Regarding the distortion I have to agree with the rolling ball characteristic...it is there, as well as the issue of glare. All in all I thought the 8X42 and the 6.5X42 are the best of the lot, the EP hiding the blemishes you mentioned.

Do you think he got a bad sample?

Andy W.
 

Canip

Well-known member
Henry,

Thank you for your - as always - thorough and enlightening post!

Regarding the roof edge, you wrote:
„ .... this roof edge shows up as fairly extreme spikes perpendicular to the line of the roof edge radiating from any small bright light source in a dark field like a street light. I noticed that Allbinos mentioned the spikes in their overview. .....
I would be interested to know if others see this same thing in their Kowa’s....“

My 6.5x32 exhibits those very strong spikes mentioned, and the roof edge is also clearly visible when viewing through the tubes from the objective side. But:
Funny detail: in all other binoculars that exhibit spikes from the roof edge, the appearance when seen with both eyes is that of a cross. In my Kowa, the spikes in both tubes are parallel, so that when viewing with both eyes, I get the impression of one prominent diagonal parallel DOUBLE spike, not a cross. Doesn‘t seem to make sense to me.

Canip
 

henry link

Well-known member
Hi Canip,

Thanks for the kind words. I noticed the same thing about the orientation of the roof lines in the two sides. In every other binocular I've seen the prisms are installed to be a mirror images of each other, so (depending on the IPD setting) one side will have the diagonal of the roof line going through from 10:00 to 4:00 and the other side will be 2:00 to 8:00. Look through both sides at once and you see a cross. In the Kowa I tested the roof edges of both sides went through at about the same angle you see in the photo I posted, but they were not perfectly parallel.

Apparently, in the Kowas the prisms are dropped in at about the same angle of rotation in both sides. I guess there's no real optical disadvantage to doing it that way.

Henry
 
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Brink

Well-known member
I see the same effect in both the 6.5x32 and the 8x32. The prism edge only cuts into the exit pupil on the 6.5x but not the 8x. I see the cross effect on the 6.5x but not the 8x. This is because the roof line is straight up and down on the 8x (12:00 to 6:00), creating a horizontal spike in both tubes. The 6.5x roof lines are about 10:00 to 4:00 on the left and 8:00 to 2:00 in the right.
 

Canip

Well-known member
Thank you both, Henry and Brink!

But, if I understand correctly, that would mean that Kowa drops the prisms in not consistently always at the same angle?
Is anybody else doing that (I understand, also from a remark of Holger Merlitz, that the choice of angle has no real optical disadvantage)?

Canip
 

henry link

Well-known member
I confess I've never given the subject any thought, but now that I am I can't think of any reason why the angle of the roof line should matter. I have a couple of roof prism monoculars. The angle of the roof line is determined by the random accident of how I happen to pick up the monocular each time it's used.

I suppose I always assumed that the interior of the binocular body was shaped so that the prisms could only be inserted one way, but that's obviously not true of BD II-XD Kowas. I can see one possible problem arising in the case of the binoculars with prism edges intruding into the exit pupil like the Kowa's. If the prism were installed so that the intruding prism edge happens to be near the bottom of the FOV that would lead to increased visible glare under typical field conditions compared to another unit with the intruding edge positioned at the top of the FOV.
 

james holdsworth

Consulting Biologist
Proof that folks that really can't see CA really shouldn't comment on such in reviews.
I've used some of these reviews to make buying decisions of sight-unseen bins in the past and, in some cases, have been very disappointed to find horrific CA in bins that were ''reviewed'' to be nearly CA free.
 
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Brink

Well-known member
Proof that folks that really can't see CA really shouldn't comment on such in reviews.
I've used some of these reviews to make buying decisions of sight-unseen bins in the past and, in some cases, have been very disappointed to find horrific CA in bins that were ''reviewed'' to be nearly CA free.


I see low levels of CA in the 6.5 model and medium-high levels on the 8x32. The allbinos article found the opposite. I can assure you that I see CA just fine through both and the 8x has much more even when accounting for the difference in mag. Is it possible that cheaply/poorly made binoculars just have poor quality control and that the people reporting CA-free binoculars just got lucky?
 

Binastro

Well-known member
I asked about molded components.

The answer I have at the moment is that one gets what one pays for.

There are molded components including prisms.
Aspherics are often molded.
There are plastic components.

There is now high speed polishing, which is supposed to be good, but here there is skepticism about whether this is true.

Perhaps someone here knows the percentage of elements in binoculars of various cost classes that are not made in the traditional way.
If no answer regarding binoculars, what about camera lenses?

I have seen large photocopier lenses all plastic, with very strange looking multicoating long ago.

B.
 

42za

Well-known member
Hello All,

Remember that these binoculars are made on high speed automatic production lines , I very much doubt if any hand fitting and adjustment takes place.

Thousands are made every month and taking modern lower levels of quality standards into account , the end results are quite good I think.

The Bean Counters will always accept "this is good enough for our markets" above "we make an excellent no faults product".

Sad , but this is the modern "throw-away" world.

Cheers.
 

etudiant

Registered User
Supporter
Hello All,

Remember that these binoculars are made on high speed automatic production lines , I very much doubt if any hand fitting and adjustment takes place.

Thousands are made every month and taking modern lower levels of quality standards into account , the end results are quite good I think.

The Bean Counters will always accept "this is good enough for our markets" above "we make an excellent no faults product".

Sad , but this is the modern "throw-away" world.

Cheers.

Is that really the case?
The consensus is that binoculars and even more so scopes are produced in such low quantities that most of the assembly is still manual.
Interestingly, it is the very cheapest models that appear to be the most numerous. In my dreams, these will lead the way to achieving good quality in high volume optics manufacture.
 

jgraider

Well-known member
Proof that folks that really can't see CA really shouldn't comment on such in reviews.
I've used some of these reviews to make buying decisions of sight-unseen bins in the past and, in some cases, have been very disappointed to find horrific CA in bins that were ''reviewed'' to be nearly CA free.

Seems to me that would be your problem, not the reviewer's.
 

james holdsworth

Consulting Biologist
Seems to me that would be your problem, not the reviewer's.

Some reviewers consistently see low CA in most everything they test, even when the overall consensus is otherwise. I'm saying that people that are rarely troubled by CA should mention that upfront when reviewing.
 

henry link

Well-known member
I agree with James that reports of no CA problems in a binocular are meaningless if the reviewer NEVER sees CA in any binocular, but I think nearly everyone can see CA if it's bright enough, just like nearly everyone can see the colors of traffic signals because they are designed to be bright enough for nearly everyone to see them.

What's required is a CA target designed to make color fringes vivid enough to be seen in all binoculars by nearly everyone, just like a stoplight. It's not that hard to make such a target. I use a simple one made of alternating black and white stripes viewed in bright sunlight or, in a pinch, under a bright "daylight" halogen lamp. Reviewers who claim not to notice lateral CA just haven't looked at a target like that. Once you can actually see it, then you can set up controlled conditions to determine which binoculars have more and which have less.
 
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mskb

Well-known member
Thank you for the very informative review @Henry Link. If we can have your opinions/results on a Kowa 663M scope, that would be awesome!
 

quincy88

Well-known member
Wow. Thanks, Henry Link. I learned a lot from this. I enjoyed the photos that you included to supplement your descriptions.
 

henry link

Well-known member
Thanks guys. I doubt that I'll have an opportunity to even see a Kowa 663. I'm not much interested in buying things anymore or even visiting stores. Mostly I just test new purchases for friends.
 

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