33 replies to this topic

[tmichaelbanks]

This question is directed to those who have purchased the Kowa 6.5x32 BD-II XD binoculars.

 

I've had nothing but praise for these little guys since I took them out of the box.  I have used them day and night and have really enjoyed the views.  I have read the feedback in the dedicated thread on glare and have experienced it myself on occasion under bright conditions.  However, tonight I took them out and looked at the half moon low in the evening sky.

 

I was shocked to see in the right barrel a ray the width of the moon image going from 2 O'Clock to 8 O'Clock.  In the left barrel was the opposite effect, the ray went from 10 O'Clock to 4 O'Clock.  The effect with both eyes was to see the half moon at the intersection of a humongous "X".  Again, these are not spikes, they are the width of the moon image and have the appearance of beams.

 

If those of you who own these binos could take a few seconds to look at the moon and let me know what you see I would very much appreciated it.

 

I understand all binos have compromises and the Kowas have presented the starry night sky well in addition to their virtues for terrestrial viewing.  But geez, not even my least expensive binos show the moon this way.

 

[ihf]

These are the roof lines. The Kowa has them 90 degree rotated against each other. I noticed them before on Jupiter. I tried on the moon and they are moon width, show without my glasses and rotate with the binoculars. They show with the VisionKing 5x25, but both in one direction and maybe a little less strong.

 

Don't look at the moon? I mean, it is only 6.5x magnification anyways.

[tmichaelbanks]

Thanks for the quick reply ihf, so it appears the effect is design-related and not due to poor QC.  And I understand your point about not looking at the moon of course. 

 

Do you know if this means Kowa skimped a bit on phase coatings or some such issue, perhaps reasoning that this magnification would be used mostly for terrestrial viewing?

[pat in los angeles basin]

There's a comment on the phenomena on a birdforums kowa discussion group by Henry Link "Kowa BD II 10x42 XD Optical Tests" Worth a read    Regards, Pat

[ihf]

I found the Kowa showing good contrast and I thought it did reasonably well below/next to the setting sun. Not perfect, but better than some other inexpensive S/P roof binos that I have. Strong contrast makes optical deficiencies very visible and the moon is one of these unforgiving test targets. If one wanted to watch the moon one better pay very close attention to them. I mostly find the moon too bright and too small to look at, even in the Canon 18x50.

 

I got the Kowa for watching the Milky Way in dark skies. I was very happy with the views the few times I was able to use on that target. It showed many pointy stars, not washing out into each other. Which I think means the optical defect is only along the thin roof line (on the moon the "rays" are as wide as the moon, but stars are points).

 

As for skimping on the prisms, they are a reason alpha roof binos are very expensive compared to Porros. I don't know what the price/performance frontier for roof prisms looks like. I don't think Kowa used worse prisms in the lower magnification 6.5x32, as I assume it shares everything but eyepieces with the 8x32 and 10x32. And I don't think the roof lines would show more due to prism undersizing for the wide field (would be vignetting and ghosting, I think). I know seeing is very personal. If I could ask Kowa to upgrade my 6.5x32 for extra money I would ask for a flattener first. Which in converse means I don't think they skimped on the prisms. Everything is a price/performance compromise.

[pat in los angeles basin]

Well, I tried to get a personal message to him "mail box full" and tried to post on the evaluation of the kowa bin he wrote and it's an old locked thread so here goes

"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." 

 So, apparently it's mentioned on the allbinos evaluation as well.... Regards, Pat

[ihf]

I think Henry Link's review 10x42 is very interesting and a good starting point for comparing the 6.5x32 to. That said the 10x42 is not the 10x32 which is not the 6.5x32. Lots of salt while extrapolating. Maybe they all use the same prism, maybe they don't? And if they do, and even if the 10x42 prism is undersized, what does that mean for the 10x32 or 6.5x32? And if there are better 10x42s on the market, does that mean there are also better 6.5x32s on the market?

[tmichaelbanks]

Many thanks, Pat and ihf, for your feedback on this issue.  I did read Henry Link's 10x42 Optical Test thread on BirdForum, including the interesting comments on the coarseness of the prism edge finish and the different orientations regarding prism insertion during manufacturing.

 

I think that if I had purchased the 8/10x32 or 8/10x42 model and found these effects, I would return them.  However, the 6.5x32 has a lot going for it in general and for me personally:  good ER with my glasses on, comfortable eye positioning, decent focus snap, good construction and ergonomics, hardly any discernible CA, great close focus, and of course that fabulously wide FOV.  It's also very nice for night sky scanning, especially when skies are dark.  I have enough binocular alternatives for viewing the moon and planets, so the little Kowas will stay in the stable.

 

Thanks again,

Mike

[Mark9473]

Do you know if this means Kowa skimped a bit on phase coatings or some such issue, perhaps reasoning that this magnification would be used mostly for terrestrial viewing?

It's not phase coating related but depends entirely on the precision put into grinding the roof edge as finely as possible.

I expect the top brands simply reject a lot of prisms, something cheaper brands can't afford to do.

 

In principle, if the disruption becomes less than about one quarter wavelength, the diffraction spike becomes invisible.

[pat in los angeles basin]

Is there a common method to ascertain the nature of the roof edge from  objective or eyepiece? I only have one pair of roof's (Eagle optics platinum ranger 8x32) and can not discern anything on them with regard to the roof edge.   Regards, Pat

[tmichaelbanks]

I don't know how to approach this question technically, all I can say is that I have binoculars ranging in price from $130 to $800, mostly roofs, and none of them have an effect this prominent.  Chalk it up to sample variation on the cheap guys, in that maybe I got good samples?

[tony_spina]

I have an observing buddy near me that has all 3 of the Kowa 32mm BDII.   I will ask him to test on the moon tonight 

[ihf]

I don't know how to test roof prisms fully. I would compare them on high contrast objects: setting sun, moon, Jupiter, Saturn, bright stars, street lights.

 

http://www.holgermer....de/porros.html

https://www.youtube....h?v=SIFiTohrBfo

 

Tony, this is an interesting experiment. Ask him for Jupiter too. I assume all three binos have the same prisms. If they all show the problem maybe this means the edge is not ground very sharp. But if some show it more than others, then maybe there is variation. And if only the 6.5x shows it then maybe the little triangle on the roof is intruding into the picture? (Basically a mild form of undersizing.)

[tony_spina]

I have an observing buddy near me that has all 3 of the Kowa 32mm BDII.   I will ask him to test on the moon tonight 

The results are in

 

The rays are present in all 3 magnifications.  From order of worst to least: 6.5x, 10x, 8x

 

The rays are also visible on Jupiter.   With that said the views are razor sharp.  Even the 6.5x shows lots of lunar details and this was just hand holding 

[tmichaelbanks]

Well that's certainly conclusive for me - design issue/choice it is.  And I'll add a +1 to the sharpness of the half-moon I saw right above the tree tops, the "X rays" notwithstanding.

 

Thanks very much ihf and Tony and friend.  It's nice to have a supportive, interested community like this.

[Rich V.]

Clearly the broad roof spikes in the Kowa are a result of the choice of a dull, rounded roof edge instead of a fine, sharp one.  I just read this on the Baader website regarding roof spiking artifacts in their Amici roof diagonals; this would apply to the Schmidt roof prism used in the Schmidt-Pecan as well:

 

"To be honest, there is one way to reduce these reflections: by destroying the prism edge. Even a sharp, perfect prism edge with a width of only 1/100 mm will produce reflections. But if you polish it round, there will be no spikes! Unfortunately, in the final image in the eyepiece, there will be a streak with a width of ca. 1 mm where contrast will suffer and details will no longer be visible, because they drown in the multitude of image errors introduced by this wide "bar" cutting the image in two halfes. We decided not to sacrifice the over-all contrast by giving the prism a blunt edge – even if that means that bright objects must be placed a little bit away from the center to avoid spikes."

 

Images of the roof line shown in Henry Link's post in Birdforum verify this, IMO.

 

Rich

[ihf]

Mhh, not sure how to interpret this. There is no 1mm thick streak in the Kowa. The streaks on Jupiter are thin barely noticeable lines. Stars don't show streaks, but magnification is low unlike for Baader diagonal. The streaks on the moon are moon width. I am actually reading the Baader site in support of Kowa having a sharp edge, but have not enough experience/expensive roofs to compare. Then again it discusses Amici, which the Kowa has not. And I am not sure what they mean with "reflections". It seems spikes and streaks are closely related?

 

Then again Baader states:

"This should be obvious only with very bright stars or planets, while dimmer stars are no problem. In this case, simply move the telescope a little bit so that the image of the bright light source isn't centered on the prism edge any longer."

 

AKA "Don't look at the moon."

[Rich V.]

All roof prisms generally split the light path across the roof line's edge, Amici, Schmidt-Pechan, whatever. Only the oversized roof prisms used in some BTs avoid the roof line.  By rounding the roof edge, the sharp, linear diffraction artifacts are blended away into a wider area where the linear spike may disappear but contrast is lost.  Instead of a sharp line, it's spread into a broader stripe by rounding the roof edge.

 

Here's a couple of photos Cory Suddarth sent me showing a laser directly hitting the sharp roof edge of a bino's prism.

 

Rich

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Edited by Rich V., 02 October 2020 - 03:19 PM.

[Mark9473]

Rich, I'm with ihf on this one. The symptoms described in the OP are consistent with a regular sharp roof edge.

[tmichaelbanks]

Well, being no optics expert I can only report what I see.  Two nights ago under clear skies, and with new eyeglasses with a proper astigmatism correction, I was able to make out one of Jupiter's moons fairly close in to the planet. An extremely bright, close-to-full moon brought the diagonal rays, but they didn't seem as prominent when as when looking at a waxing gibbous moon just past half.  I couldn't seem to make out anything around the moon (loss of contrast maybe?).

 

Tonight the skies look clear again, and I'll be able to look at a waning gibbous moon, Jupiter, Saturn, Mars and Vega.  With my new eyeglasses prescription I don't notice much in the way of spiking, except the broad rays on the moon.  Jupiter and Mars have a little bloat and a uniform pattern of very short spikes.  But for Vega and any other dimmer target I've looked at, no spikes, bloat or other distortions.  All-in-all, whatever the prism edge finishing, views are very nice.

[Jon Isaacs]

It's not phase coating related but depends entirely on the precision put into grinding the roof edge as finely as possible.

I expect the top brands simply reject a lot of prisms, something cheaper brands can't afford to do.

 

In principle, if the disruption becomes less than about one quarter wavelength, the diffraction spike becomes invisible.

 

At binocular magnifications.. but at high magnifications, the spikes will be there.  It's the result of the split light path..

 

Jon

[ihf]

To close the loop it was stated in the Swarovsky NL thread that the spikes are invisible in the 10x42 and 12x42 on moon and Jupiter.

[gwlee]

FWIW, I have never seen any spikes using my older Nikon 8x42 Premier roof, my new Swarovski 8x32 EL roof, or my Baader T2 90* AMICI prism used with my 72mm f6 refractor (62x) when pointed at any object. 

 

However, when I put the same AMICI prism in my 92mm f6.7 refractor operating at 87x, and pointed it at Arcturus on a vey clear, dark night to try to induce a spike, I saw a long horizontal spike, which I assume was caused by the roof prism. When I get a chance, will try to experiment to confirm it, but haven’t seen another spike since with this configuration in the course of normal observing. 
 

I am guessing that spikes are probably always there, but perhaps below my threshold of detection unless the magnification is high or the prism construction is less precise. Expect people vary in their ability to detect these spikes. 

Edited by gwlee, 11 October 2020 - 03:58 PM.

[tmichaelbanks]

Intrigued by the prism roof-line "beam" phenomena of my Kowa 6.5x32s, just for fun () I took a number of my binoculars out several nights ago to look at some bright objects: Jupiter, Saturn, Mars, Vega, full Moon, and MW star clouds in Cygnus (admittedly not very bright in my skies).  I viewed with my new eyeglasses which significantly reduced, but did not eliminate, spikiness of these objects naked eye.

 

This is a non-scientific comparison to say the least, the only constants in the "testing" being me and the sky conditions.  But I have been purchasing binoculars for different purposes over the past two years and was curious to see if any other of my units exhibited the prism roof "beams" of the little Kowas or other aberrations on bright objects.  In decreasing order of magnification with approximate retail price and exit pupil noted:

 

- Vortex Diamondback 15x56 ($350, EP 3.7 mm):  bit of bloat and small amount of CA on Jupiter, smaller bloat on Saturn, very small spikes on Vega and Mars, slight CA on limb of moon, MW clouds easily visible.  OK on planets, very good on the Moon.  No roof "beams."

 

- Vortex Diamondback 12x50 ($250, EP 4.2 mm):  bloat and small spikes on Jupiter, small bloat on Saturn, small spikes on Vega and Mars, moderate CA on limb of moon, MW clouds faintly visible.  OK on planets and Moon.  No roof "beams."

 

- Nikon Monarch HG 10x42 ($1,000 EP 4.2 mm):  no bloat, spikes, or CA on any targets, good definition of MW star clouds.  Aesthetically, gave the best overall views and perceived sharpness.  No roof "beams."

 

- Nikon Monarch 5 8x56 ($520, EP 7.0 mm):  small spikes on Jupiter, Saturn, Vega and Mars, just a trace of CA on the moon, good definition of MW star clouds.  Very good overall on bright objects, spikiness perhaps due to large EP and my eyes.  No roof "beams."

 

- Oberwerk 8x32 HD II ($200, EP 4.0 mm):  minor bloat and small spikes on Jupiter, small spikes on Saturn and Mars, but curiously no spikiness on Vega, slight CA on the Moon, MW star clouds faintly visible.  Good views on stars, not as good on brighter objects, perceived sharpness on moon worst of this particular set, but not bad overall.  No roof "beams."

 

- Kowa BDII32-6.5XD ($350, EP 4.9 mm):  small spikes on Jupiter, Saturn and Vega, very faint but detectable roof "beams" on Mars, prominent roof "beams" on the Moon but very good perceived sharpness, very good definition of MW star clouds with full moon at roughly the same elevation as Cygnus and about 90 degrees in azimuth away.  No CA to speak of.

 

Well, I don't know if there's many cosmic conclusions to be drawn from these bright-object observations, except maybe that optically you get what you pay for (i.e., with the two Nikons) and that the Kowas generate these prism roof beams when an object reaches a certain brightness threshold.  Despite the "beams" the Kowas do provide satisfying views and the perceived sharpness is very good, day or night.  Also, the nearly complete absence of CA is a real plus of these binoculars.  I have no idea whether this is due to the objective design (a triplet seems unlikely at this price point, but I'd love to know) or if it might be due to the lower magnification.  Either way, this is icing on the cake for a lightweight, well-built, very wide FOV binocular.

[Mark9473]

Nice investigation. I like the 'roof beams' terminology.

It would be interesting to repeat this on a near Full Moon, that's really a severe test in my experience.