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Review of Kowa BD II 6.5x32 XD (1 Viewer)

Gijs van Ginkel

Well-known member
Canip, post 100,
That is clear, but by eye I did not see the aberration you and Henri describe. That does not mean that I do not believe you both.
Gijs van Ginkel
 

henry link

Well-known member
Gijs,

I think you will be able to see the distortion if you examine a 5 cm circle from about 8 meters away (or any combination of circle size and distance that results in a circle that subtends around 2-4º of AFOV).

Move the circle from the field center to the edge. In the Kowa and other binoculars with large amounts of angular magnification distortion in the outer FOV the circle will compress to an oval as it nears the edge of the field. If you move the circle quickly back and forth between center and edge I think you'll see the shape changing back and forth between circle and oval.

Henry
 
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Gijs van Ginkel

Well-known member
Henri,post 102,
It is almost 11 oçlock here now in the evening. Our living room is about 10 m long. I put a circular object of the size you suggested at the distance you suggested from the point of observation (my wife seriously started to doubt my mental health already) and I searched for the distortion you described. I tried very hard, but the circle stayed a perfect circle when I move the circle over the FOV.
There are now two possibiliteis:
-1-my wife is right
-2- the sampe I have for the test does not show the distortion
Gijs van Ginkel
 

henry link

Well-known member
Henri,post 102,
It is almost 11 oçlock here now in the evening. Our living room is about 10 m long. I put a circular object of the size you suggested at the distance you suggested from the point of observation (my wife seriously started to doubt my mental health already) and I searched for the distortion you described. I tried very hard, but the circle stayed a perfect circle when I move the circle over the FOV.
There are now two possibiliteis:
-1-my wife is right
-2- the sampe I have for the test does not show the distortion
Gijs van Ginkel

That's very surprising to me, Gijs. As you know it's impossible for a binocular to have no distortion. The designer has to choose between more pincushion with less angular magnification distortion, more AMD with less pincushion or the mixture of the two called "mustache" distortion. One would expect designers to choose the same mix of distortions within a model series and I can't think of any way that distortion could be sample dependent.

If there is essentially no angular magnification distortion in the outer part of the field of your 6.5x32 then it follows that there must be a healthy dose of pincushion distortion in that area because one distortion increases as the other decreases.

If you're willing to try my next suggestion your wife will no longer be in any doubt about your mental health (mine got used to such behaviors a long time ago). If you examine a set of parallel lines or a grid pattern (about 0.5 meters from the binocular) by looking through the front of the binocular instead of through the eyepiece you'll see a pattern like the photo below of window blinds from my 10x42 tests. If the distortion of your 6.5x32 is like the 10x42 I tested it will show the same wavy pattern you see in the photo with barrel distortion increasing out to about 60% of the circle's radius and then reversing and straightening out from there to the edge. If you see a smooth increase in barrel distortion all the way to the edge of the circle that would indicate increasing pincushion all the way to the edge of the FOV when viewing normally through the eyepiece and would match what you saw when you observed the small circle at the field edge.

Henry
 

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Canip

Well-known member
Henry / Gijs,

this is what I got on the 6.5x32 today with my modest photgraphic skills.
What do you make of it?
The "mustache" is much less pronounced than I anticipated, there is just a hint, but I had trouble getting everything up to the field stop into the picture, so perhaps there is more than what is visible in the image.

Canip
 

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henry link

Well-known member
Hi Canip,

I think the camera was too far from the front of the binocular. It needs to be close enough so that the camera can see the sharp field stop of the eyepiece. The fuzzy opening in your photo appears to be an out of focus internal stop, probably a prism aperture.

I use a DSLR with the kit zoom set to 55mm f/5.6 and placed as close as possible to the binocular objective. Auto focus on the blinds is good enough to also bring the eyepiece field stop into sharp focus.

Henry
 

Torview

Well-known member
I don`t have a blind so tried this................
 

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Canip

Well-known member
Thanks, Torview, well done, much clearer result than in my attempt, I think this is what Henry was looking for.

Canip
 

mwhogue

Registered User
Supporter
That's very surprising to me, Gijs. As you know it's impossible for a binocular to have no distortion. The designer has to choose between more pincushion with less angular magnification distortion, more AMD with less pincushion or the mixture of the two called "mustache" distortion. One would expect designers to choose the same mix of distortions within a model series and I can't think of any way that distortion could be sample dependent.

If there is essentially no angular magnification distortion in the outer part of the field of your 6.5x32 then it follows that there must be a healthy dose of pincushion distortion in that area because one distortion increases as the other decreases.

If you're willing to try my next suggestion your wife will no longer be in any doubt about your mental health (mine got used to such behaviors a long time ago). If you examine a set of parallel lines or a grid pattern (about 0.5 meters from the binocular) by looking through the front of the binocular instead of through the eyepiece you'll see a pattern like the photo below of window blinds from my 10x42 tests. If the distortion of your 6.5x32 is like the 10x42 I tested it will show the same wavy pattern you see in the photo with barrel distortion increasing out to about 60% of the circle's radius and then reversing and straightening out from there to the edge. If you see a smooth increase in barrel distortion all the way to the edge of the circle that would indicate increasing pincushion all the way to the edge of the FOV when viewing normally through the eyepiece and would match what you saw when you observed the small circle at the field edge.

Henry

Guys,

Following Henry's instructions I see a view of blinds more like his photo in post 106 than the others posted. The distortion in mine is much more pronounced at the top of the image at 0.5 meters, i.e. the blinds appear much more curved at the top than at the bottom. Adjusting focus lessens the distortion without affecting the center of the image. But when I increase the distance from the blinds a bit the distorted area decreases in size and begins to look more like pincushion.The top blinds appear much less curved. The distortion mostly appears in the shape of a flare at the top rather than a complete circle around the image.

If I recall correctly, another member described a similar asymmetrical distortion pattern in an earlier post.

Mike
 

MandoBear

Well-known member
I can't help wondering whether there is any reliable correlation between the distortions seen when looking through a binocular from ocular to objective and those when looking from objective to ocular. I appreciate that the same lenses are involved but the distances between the object, the eye, and the respective lenses of the binocular are very different in each scenario, so I'd be wondering if the image formed inside the binocular is in the same plane?
 

henry link

Well-known member
Thanks Canip, Torview and Mike for your efforts!

Torview's image does show a more complete view of the mustache distortion pattern, but I'm curious about the prism edges that visibly intrude inside what I assume to be the eyepiece field stop. Notice that the prism edges in my photo of the 10x42 are comfortably clear of the field stop. I've never seen that before, so I'm not sure what it means. Are the prisms simply too small for the size of the field stop?

Mike, I think the distortion is almost certainly symmetrical, but it can appear asymmetrical with just a little tilt of the binocular. I wasn't able to move the binocular much farther than 0.5 meters from the blinds without the field stop circle becoming wider than the width of the blinds. Other kinds of targets using single lines moved across the field can be found for longer distances like door jams, the line between the wall and ceiling or building edges or roof lines at greater distances outside.

Mando Bear, I've tried this with many binoculars. So far there has always been an exact reversal of the distortion type compared to normal viewing. The advantage of this method for photography is that the pattern of lines within the field stop circle is small enough in the photograph so that its distortion doesn't mingle with whatever distortion the camera lens develops outside that small central area.

It would be interesting if you guys with 6.5x32s could try the small circle experiment that Gijs did earlier. I think that's easier and it allows for a normal, through the eyepiece, observation of the angular magnification distortion at the edge of the field.

Henry
 
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Brink

Well-known member
The view through the objective on mine looks very much like Torview's picture.

Through the eyepiece I had a similar experience to Gijs. I think I can see the angular distortion when the circle gets very very close to the field edge, but it is certainly not as pronounced as in Henry's pictures
 

mwhogue

Registered User
Supporter
Henry, Post 115, I am not at all technically inclined but hope to correctly or at least helpfully explain the following:

Paying more careful attention this evening as you suggest to the angle of the binoculars when viewing blinds at 0.5 m, you are right I see much less angular distortion and more pin cushion distortion. However the pattern of the distortion is still in a narrow flare shaped area between 9 and 3 o'clock with no noticeable distortion elsewhere even including on the very edge. In comparing what I see in your picture post 106, in my 6.5 I see (what I understand are) prisms slightly intruding into the image at 4, 5, 11 and 1 o'clock. In contrast, in your picture 106 I see prisms outside the field stop at 2, 4, 7, and 8 o'clock. In connection with some of the discussion in your thread on the 10x42 model regarding the possibly random or unusual placement of prisms in the series, I wonder whether the prism placement with edges clearly visible within the image in mine at 11 and 1 o'clock -pointing directly at 12 O'clock High- could explain distortion appearing only at the top of the field in my sample on the blind test.

Tomorrow I will try to conduct some of the other tests mentioned and use the 6.5 in the field with particular attention to correlating distortion in actual use with what my tests and various photos posted by others show.

Mike
 
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mwhogue

Registered User
Supporter
I just thought of something: turning the binoculars upside down and with the two closely positioned prisms pointing directly at 6 rather than 12, I still get distortion only at the top of the image. D'OH!

Mike
 

henry link

Well-known member
From the observations and photos posted so far I would say the 6.5x32 has the same mustache distortion profile as the 10x42, but the swing between the two rectilinear distortions is less extreme in the 6.5x32.

From my photo of the blinds you can see that the 10x42 develops pretty strong pincushion (looks like barrel in the photo) out to about 60-70% of the distance from the center to the edge and then reverses so strongly in the outer part of the field that the the last line at the top of the photo actually shows very mild barrel distortion (looks like pincushion) if you place a true straight edge next to it. In my photo of small circles taken through the eyepiece of the 10x42 the circle at the right edge of the field is so compressed by AMD that its vertical diameter is about 11-12% greater than its horizontal diameter. Torview's photo of the 6.5x32 shows less pincushion at 60-70% and the reversal is weaker so that there is still just a little pincushion at the field edge. Based on that pattern I would expect less AMD at the field edge and a less noticeable Globe Effect when panning with the 6.5x32.
 
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mwhogue

Registered User
Supporter
To close the loop:

Door jam test showed distortion just like the pic in Torview 111.

Small Circle test sometimes seemed to show very slight elongation at the outer 10%.

In actual use I could induce some globe effect when scanning. Not as noticeable as several SVs but I am not bothered much by GE.

Mike
 

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