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What's it called when the center of the image is a brighter circle? (1 Viewer)

18000bph

Well-known member
In certain bright conditions some binoculars present a noticeable effect of a sharply defined bright circle on the inner part of the image (roughly inner 2/3 of the total image). I am trying to figure out what this condition is called.

I am also interested in discussion on what causes it, certain brands or models that are worse than others, ways to check for it when trying out retail binos, etc.

Thanks.
 
Sweetspot or on-axis? The area in the center of the FOV that is in sharp focus and then depending on the binoculars blurs more as you approach the edge.
 
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Hello 18000bph. Just to clarify, are you describing a noticeable drop in brightness or sharpness? Or both? Binastro's and Dennis's posts respond to each. Another possibility for a drop in brightness is the issue of field illumination. It is pretty normal for the light transmission at the edge of the field to be lower than at the center. But field illumination typically drops gradually towards the edge and is not usually noticeable to most users.

Perhaps if you listed few models with which you've encountered this phenomenon, then somebody with the same model can try to comment on your observations.
 
Hello 18000bph. Just to clarify, are you describing a noticeable drop in brightness or sharpness? Or both? Binastro's and Dennis's posts respond to each. Another possibility for a drop in brightness is the issue of field illumination. It is pretty normal for the light transmission at the edge of the field to be lower than at the center. But field illumination typically drops gradually towards the edge and is not usually noticeable to most users.

Perhaps if you listed few models with which you've encountered this phenomenon, then somebody with the same model can try to comment on your observations.


Thanks for the responses everyone.

I'm describing a ring around the edge of the image that shows an abrupt drop in brightness from the center... maybe 50% as bright. Think of an eye in low light where the pupil has expanded--the blue or brown iris would be the dimmer part and the black pupil would be the full brightness portion in similar size ratio. It is completely unrelated to image sharpness and the change is very abrupt. The dimmer outside ring is noticeable in the exit pupil as you pull the binoculars away from your face. Perhaps I can take a picture next time the conditions are right.

Vignetting is probably it and I don't have a good understanding of the concept. I had thought that vignetting was more of a blackening of the edge. The effect in question is similar in intensity to the squaring off of the exit pupil with BK7 porro prisms so vignetting is probably it.

The binoculars that brought about the questions are my new Kowa Genesis 8x33. The condition is not present during normal use but is quite apparent when it's there. The dimmer portion is still quite usable, it's just a distraction. I also see it on occasion with my Celestron Nature DX 10x42 though less intense. I've never noticed it with Leica UV 10x25 or Leupold Yosemite 8x30 but perhaps the exit pupils are too small on those. I don't have enough experience with anything else.
 
Thanks for the responses everyone.
The condition is not present during normal use but is quite apparent when it's there.

You've told us when you don't see it, so can you tell us when you do see it?

You say "when conditions are right" but we don't know what those conditions are.
 
You've told us when you don't see it, so can you tell us when you do see it?

You say "when conditions are right" but we don't know what those conditions are.

I agree we don't have enough information on this aspect but the OP has said he notices this in the exit pupil as he pulls the binos away from his eyes. Since he also said he doesn't see this phenomenon in normal use it sounds like he is more curious than complaining.

Lee
 
OK I have pulled out my Genesis 8x33 and I can see what 18,000 is referring to. If I hold the binos about 8 inches away from my spectacles and aimed at todays sky covered in brightish pale cloud, I can just make out a central disc that appears to be brighter than the surrounding annulus. I say appears to be brighter because the annulus seems to have a kind of texture to it whereas the central disc appears smooth and clear almost as if the centre is in focus (but there is no image) and the annulus isn't. I am wondering if I am seeing some kind weird representation of the binos' sweet spot and the surrounding area that is not so sharp. But viewing a bino from well beyond the exit pupil position may create all kinds of phenomena that are not relevant to normal usage.

Anyway as I bring the binos closer to my eyes this image remains constant (doesn't change when I rotate the focus wheel from one end to the other) until the two fields of view from the two tubes begin to merge into the single field and at which point the effect totally disappears. As 18,000 says, in normal use it it not there.

I also tried this with a Leica Trinovid HD and the same effect could be seen through this too.

Lee
 
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I had a brief opportunity to use the Kowas in very bright conditions today and was able to produce the effect. I discovered that it is related to my pupils being too far from the eyepieces. I adjusted the eyecups flush with the eyepieces and mashed them as far as I could get into my eye sockets. The bright circle is gone... slowly pull the bins away and it returns.

Anyway, thanks for the responses. I asked the question because I was curious about the effect and I assumed the veterans would have seen it all. Apparently this particular phenomena is highly personalized.

When the "rings of brightness" are present it in no way deteriorates the image quality of either bin, it's only a minor distraction. I am learning to love these Kowas for their strengths and I think very highly of the Celestron in the low budget segment.
 
Anyway, thanks for the responses. I asked the question because I was curious about the effect and I assumed the veterans would have seen it all. Apparently this particular phenomena is highly personalized.

It certainly was hard to see for me but once I had seen it in the Kowa it only took a moment to find it in the Leica. Perhaps it is present in all binos but in all normal viewing through both Kowa and Leica it is not there at all.

Lee
 
Q: What's it called when the center of the image is a brighter circle?
A: Bad view that occurs in bins (and scopes) when held too far from one's eyes.

--AP
 
In certain bright conditions some binoculars present a noticeable effect of a sharply defined bright circle on the inner part of the image (roughly inner 2/3 of the total image). I am trying to figure out what this condition is called.

I am also interested in discussion on what causes it, certain brands or models that are worse than others, ways to check for it when trying out retail binos, etc.

Thanks.

I was awakened one night at college by frenzied friends who, knowing I was into amateur astronomy, insisted I come out and see the UFO. The clouds were swiftly moving past JUPITER and, having no experience in such matters, nature duped them half as much as they were duping each other.

Those new to ANY profession, hobby, or pastime are prone to experience things that are really not problems and that more experienced observers have seen so often, the anomaly is taken for granted. It’s all part of a healthy learning curve.

I believe this is a situation veiling glare. If the aperture of the field stop is too big light can get to the retina ... that shouldn't :cat:

Finally, WELCOME!

Bill
 
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