33 replies to this topic

[tmichaelbanks]

Hi Mark, I did this comparison a couple of weeks ago when the moon was full.  The "beams" were quite prominent in the Kowa and form a symmetric "X" pattern, in my unit 10 O'Clock to 4 O'Clock and 2 O'Clock to 8 O'Clock.  It would be interesting to be able to examine the edges of the edge of the prisms to confirm that's the source.  Unfortunately, I don't have a spare unit to donate to the effort...   

[Rich V.]

Wide "beams" just seems wrong to me in a proper binocular.  My only small roofs currently are my Pentax 10x43 SPs and they don't show a trace of spikes or beams on the Moon or bright planets that I've ever seen.  I'd expect a spike, perhaps, from a roof under some conditions but the "beams" on the Moon would be another thing altogether.  I'd think they would affect contrast day or night even if they were not immediately apparent.  

 

Rich

[Mark9473]

No Rich, a spike can only happen on a point source. With a larger light source the diffraction effect takes up the full width of the light source.

[Rich V.]

Mark, that's why I referred to both "spikes" and "beams" as referred to in tmichaelbank's post above. He said that the "beams" were quite prominent.  If a point source striking the roof line creates a diffraction spike, wouldn't a line of points the width of the Moon passing over that line smear out into a "beam"?  I'd think it would be a poor roof edge that made a "beam" of diffracted light bright enough to be prominent, though.  I've read that in some applications, a roof prism's roof edge is actually intentionally rounded, not ground sharp, to diffuse the diffraction spike effect.  Not sure what that application would be but hopefully not Kowa binoculars.    

 

I've never looked through this Kowa bino, so I'm just commenting based on responses posted above.  You're certainly welcome to disagree and I'll do my part to keep an open mind.   

 

Rich

 

 

[Mark9473]

Rich, I think you might have either very limited experience with roof prism binoculars, or else only with the very best of them.

[Rich V.]

Well, you need only look at my profile to see that I only have three roof prism binos listed and two don't count.  Pretty much a Porro guy as it turned out.  I've had the Pentax 10x43 DCF SP for years now but have never seen the kind of artifacts described above through it.  I use a Swaro 80mm roof spotter on the night sky from time to time and am impressed how good stars look even at 60x so I know a good roof prism is possible to make for a price but I don't know where the lower limit lies.

 

Rich

[Jon Isaacs]

Just something to keep in mind:

 

Stars show diffraction spikes.

 

Extended objects like the moon show diffraction bands that are the sum of the diffraction spikes over the area of the extended object.

 

Jon

[tmichaelbanks]

Minor coda to this thread, allbinos.com has a "First Impressions" article out today on the Zeiss SF 8x32 and 10x32:

 

https://www.allbinos...dex.php?art=182

 

Toward the end of the article are a pair of photos comparing nighttime bright sources (streetlights) in the 8x32 and 8x42, looking through one barrel.  The 8x32 has spikes from the edge of the roof prism similar to my little Kowas and I have to wonder how the 8x32s will do on bright extended objects like the moon.

 

At well over $2,000 USD that just doesn't seem right, but perhaps it's not an issue for the birding market.

[ihf]

That's what I see as well. Wikipedia shows the single streak under dirty optics. Not sure that I agree, as we have 2x2 lenses already. But I am wondering why the SF 8x32 shows it and the SF 8x42 not. If there is no sample variation then it must be down to construction. Not of the prism, but something else. And if it is construction, then (just speculating) maybe the placement of the prism relative to the image plane could have the effect? Well, I doubt hunters or birders use an 8x32 at night. So I guess Zeiss got that one right (if it was intentional).