Sweet spot peculiarities; I can´t get the center as sharp as the periphery (1 Viewer)

Hello!

I have noticed when looking through my binoculars, and I have a bunch, that the sweet spot, to my view at least, is, to begin with, not symmetrical around the optical axis; it is much wider horizontally than vertically.
But then is the other oddity; I can not get the center as sharp as I can get a field that (along the horizontal axis) is positioned nearer to where the edge fuzziness begins. As the sweet spot is not a circular area, the location of these zones of increased sharpness is not a concentric ring, rather occurring in two (or one) sector(s) right and/or left of the central (let´s say) 50% of the view. It differs slightly between the binoculars I have tested:

Granite 9x33: Significant difference between center sharpness (roughly 50% of area) and two zones to the right and left at approximately 60% of distance from center to the edge. I can get, for instance black text on white background, pretty sharp in the center area. But if I then move the binocular so that the same text comes closer to the edge, the text get super sharp (If I had that sharpness at the center I would need no alfa bin! The sharpness here beats my HDs hands down). I have seen the same in two specimens of the Granite 9x33. Further out the image gets fuzzy pretty fast. Overall, the Granite has a sweet spot that feels similar to my HD and better than my Dialyt.

Zeiss Dialyt 7x42 TP: I notice slightly the same effect as for the granite but not as strong. Image gets just a little bit sharper just before the edge fuzz begins (a slower transition than in the Granite and HD).

Zeiss Conquest HD 8x32: I notice the same amount of difference as in the Dialyt, but only at the left side (!).

Bushnell Ultra HD 10x36: About the same difference as with the Dialyt.

Jaxy 12x56ED 96m FoV (I think similar to the B&S Savannah): I do not notice the effect, it is sharpest in the middle and gets fuzzier towards the edge.

I have tried to find the answer in the following thread: "Sweet spot size a function of focus accommodation?", but from what I understand the difference they discuss there is due to field curvature, and they can actually get the center as sharp as near the edge by adjusting the focus and eye accommodation. I am not able to get the center of the field as sharp as the zones nearer to the edge whatever I do.
If it had been just in one binocular I would have thought it was a bad specimen. The difference in my Granites is so big that I get annoyed (well, the center is pretty good for that price), I now tend to position birds etc far off center to get that little extra sharpness! But it feels strange to look at objects not straight on.

Why is the sweet spot wider horizontally than vertically?

What causes the sharpnes to be better off center than at center?


I should add that I have pretty good eye sight, I do not wear glasses, and I am in the mid 40´s.

Sad to say your eyesight may not be as good as you think. Identifying letters on a chart doesn't really test for subtle defects in eye geometry. If you haven't gotten past the eye chart and into refraction then no one has really tested the structure of your vision. Short of a visit to eye care professionals I'd try this.

Make sure the binoculars are secured by a tripod or some such and then test each barrel of the instrument separately with each eye separately. So left barrel with right eye, left eye. Right barrel with left eye, right eye. Leave both eyes open when you're testing but cover the objective of the barrel you're not testing.

I strongly suspect you're going to find that the effect you identify is a function of which eye you're using, not which barrel. This would mean that the binoculars are fine but one of your eyes isn't up to par.

Best of luck with it,
Jerry

can i ask , when did you last have an eye test ( if ever ). you may think you have good eyesight but when we hit our 40's our eyesight can change considerably and degrade but we may not notice it and compensate naturally without awareness. if the issue was with just one pair binoculars you could point it towards a problem , but across all pairs then its worth looking at other possible reasons. never any harm to get your eyes checked, we only get one pair and as someone whos had to wear glasses since 3 years of age i know how important it is for me to get regular checkups and around age 43 i had to switch to varifocals as single vision glasses didnt cover a wide enough range for me any more.

. It also seems to me to be related to your eyes rather than the binoculars.

When you get your eyes tested, try to get a professional who is prepared to give you exhaustive tests. Perhaps somebody could recommend a suitable person.
Besides the geometry of the eyes, the general health of the eyes should be tested.

What do you see when you look, without optical aid, at the planet Venus or Jupiter, both of which are bright at the moment?

I got focused on the binoculars and missed the forest for the trees. Thanks to Dean and Binastro. Yes, by all means, get a medical assessment of your eyes. Problems with your central vision can be an early sign of a very serious problem. This isn't something to fool about. You want an MD, an ophthalmologist to examine you. A simple eye chart is really, really not good enough in your situation.

Jerry

I think you have misunderstood. I do not have a sharper periphery of the field when I center my eyes along the optical axis, i.e. look at the center of the field. It is when I move the field of the binocular so that its peripheral part of the view comes over the object that it gets sharper.
The binoculars are not some kind of spectacles. If I am unable to see sharp in the middle of the view I should also be unable to see sharp when I direct my eye to another part of the field, i.e. if I supposedly have a fuzzy central part of my vision it would be fuzzy wherever I direct it.
Personally I think it is more related to something I read in a thread where the Kowa Genesis 8x33 was discussed and where someone said that he had noticed how the sharpness varied from being sharp in the center, then slightly fuzzier, then again sharp before the final drop of edge sharpness.

In case you are just trying to make fun of me because I have expressed myself in some less succinct way, then I will cancel this thread and leave the forum.

BTW, I do not recognize any of you as those that regularly discuss optics on this forum.

613Orm said:

I think you have misunderstood. I do not have a sharper periphery of the field when I center my eyes along the optical axis, i.e. look at the center of the field. It is when I move the field of the binocular so that its peripheral part of the view comes over the object that it gets sharper.
The binoculars are not some kind of spectacles. If I am unable to see sharp in the middle of the view I should also be unable to see sharp when I direct my eye to another part of the field, i.e. if I supposedly have a fuzzy central part of my vision it would be fuzzy wherever I direct it.
Personally I think it is more related to something I read in a thread where the Kowa Genesis 8x33 was discussed and where someone said that he had noticed how the sharpness varied from being sharp in the center, then slightly fuzzier, then again sharp before the final drop of edge sharpness.

In case you are just trying to make fun of me because I have expressed myself in some less succinct way, then I will cancel this thread and leave the forum.

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I assure you, no one is making fun of you. Since you experience the problem with several binoculars from different manufacturers the only consistent variable is your eye. Sincerely, I think I speak for all the other posters when I say we expressed a genuine concern for your sight.

So, I repeat, mount the worst of the offending binoculars and systematically check each barrel and each eye. What's going on is an empirical question that can only be answered with testing. If there's a problem with the binocular this will narrow the possibilities, if there's a problem with your eyes this will tell. We can speculate forever if we like but only tests will offer a real answer. Try it and let us know the results.

Jerry

As you can read in my first post, the Jaxy 12x56 is perfectly sharp right at the center. The same with a pair of B&S 8x42 Sierra that I have access to. In case I had an eye sight problem also these binoculars would give me a fuzzy view. I would then also not be able to see any other part of the field of view in the binocular sharp when directing my eyes to different parts of the view (The center is supposed to be the sharpest part of the sweet spot).
I will do the barrel test.
But I urge others to take their binoculars, focus the center as good as possible on, for instance, a tree trunk with detailed bark, then slowly move the view of the binocular so that the tree trunk moves towards the periphery of the field while keeping the eyes focused on it (More or less the same as everyone does when checking the size of the sweetspot every time they grab a new binocular). Do not touch the focus screw of the binocular, it should keep the focus as it had when the trunk was at the center.

I am not sure if the binoculars that show the strongest effect (the Granite) has a field flattener. If that somehow can have an influence. The Granite and the HD are the ones that have the widest sweet spot of the binoculars I have tested. The ones with the smallest sweet spot are the ones with the least effect.

6130rm,

The observations in your first post could be mostly explained by a combination of astigmatism in your eyesight and off-axis astigmatism in the binoculars.

All binoculars have to some amount of astigmatism toward the field edge with the axis of the astigmatism radiating from the field center like the hands on a clock. When combined with astigmatic eyesight that means that the binocular's off-axis astigmatism along one radiant (perhaps in this case the horizontal axis) can act to correct eyesight astigmatism in an area along the axis where the amount of astigmatism in the binocular matches the amount in the eye. The radiant at a right angle to that one (in this case the vertical axis) will then unfortunately combine with and reinforce eyesight astigmatism. The result will be poor off-axis sharpness toward the top and bottom of the binocular field and two areas of best sharpness toward the 3 and 9 o'clock positions of the field edge. This scenario assumes zero astigmatism in the field center which then isn't as sharp as the off-axis spots because there's no compensating astigmatism in the center to correct the eyesight astigmatism.

Henry

613Orm said:

I think you have misunderstood. I do not have a sharper periphery of the field when I center my eyes along the optical axis, i.e. look at the center of the field. It is when I move the field of the binocular so that its peripheral part of the view comes over the object that it gets sharper.
The binoculars are not some kind of spectacles. If I am unable to see sharp in the middle of the view I should also be unable to see sharp when I direct my eye to another part of the field, i.e. if I supposedly have a fuzzy central part of my vision it would be fuzzy wherever I direct it.
Personally I think it is more related to something I read in a thread where the Kowa Genesis 8x33 was discussed and where someone said that he had noticed how the sharpness varied from being sharp in the center, then slightly fuzzier, then again sharp before the final drop of edge sharpness.

In case you are just trying to make fun of me because I have expressed myself in some less succinct way, then I will cancel this thread and leave the forum.

Click to expand...

a huge overreaction to people offering some genuine consideration. especially since having a family history of eye problems ( cateracts, macular degeneration and glaucoma ) and myself suffering from longsightedness i often have had difficulty with binoculars and am fully aware and conscious of even the slightest change in my vision and so some of the issues you stated in your original post could quite easily relate to an issue with the eyes.

quite offended that you think people were making fun when they were actually trying to help. and just because you dont recognize us from other posts has little to do with anything, other than people actually trying to help.
you might want to be consider your words more carefully next time you ask as if your going to have a poor attitude then people wont bother trying to assist

I wonder if it is actually the contrast that somehow peaks towards the edge rather than the resolution. It seems the black letters on white background get blacker in that region than at the center, this may give the illusion of a sharper image.

henry link said:

6130rm,

All the observations in your first post would be neatly explained by a combination of astigmatism in your eyesight and off-axis astigmatism in the binoculars.

All binoculars have to some amount of astigmatism toward the field edge with the axis of the astigmatism radiating from the field center like the hands on a clock. When combined with astigmatic eyesight that means that the binocular's off-axis astigmatism along one radiant (perhaps in this case the horizontal radiant) can act to correct eyesight astigmatism in an area near the edge of the field where the amount of astigmatism in the binocular matches the amount in the eye. The radiant at a right angle to that one (in this case the vertical radiant) will then combine with and reinforce eyesight astigmatism. The result is poor sharpness toward the top and bottom of the binocular field and two areas of best sharpness toward the 3 and 9 o'clock positions of the field edge. In this situation the field center isn't as sharp as those off-axis spots because the binocular doesn't have any compensating astigmatism in the center to correct the eyesight astigmatism.

Henry

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Yes Henry, that was my thinking with the binocular test likely to reveal which eye is the culprit since astigmatism is rarely bilateral and symmetric. Now that I'm older and have witnessed a lot things that weren't "likely" destroy people's lives I tend to act with an abundance of caution, but astigmatism is most likely by far. Presumably your 3,482 posts will lend reassuring credibility to the proposition.

Best,
Jerry

DeanMessenger
Sorry to have offended you. I felt I got strange replies that seemed not to have read my first post, i.e. that I do not notice the effect in all tested binoculars. Indeed, my reaction was to feel offended myself.
However, the post from Henry Link seem to confirm it can be a combination of eyesight problem and astigmatism of the binoculars. Interesting that it could combine so that the poorer the quality of the binocular the sharper the image could get, unfortunately off center.

613Orm said:

DeanMessenger
Sorry to have offended you. I felt I got strange replies that seemed not to have read my first post, i.e. that I do not notice the effect in all tested binoculars. Indeed, my reaction was to feel offended myself.
However, the post from Henry Link seem to confirm it can be a combination of eyesight problem and astigmatism of the binoculars. Interesting that it could combine so that the poorer the quality of the binocular the sharper the image could get, unfortunately off center.

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Although far, far from typical - I've also seen mis-collimation result in similar problems. This is why it's good to test, so we know for sure.

Henry's account might explain it all, but depending on how you are looking through the bin off-axis, I think you might also be getting some improvement through vignetting (effectively masking some of the objective).

--AP

I went straight home and did the barrel test with the Granite. But I fear it did not give much. Maybe maybe there is a small effect on the right hand side in the left barrel when I look with the right eye. Infact, they now appeared pretty sharp in the center both barrels with either eye. But if something wrong with the binoculars I think it should have been a constant error. The effect is more clear when looking with both eyes (still subtle I must point out. We are talking fine differences I doubt most would even care about*), but strangely it seems to vary with the light, maybe a contrast thing. Not sure if that can vary towards the periphery. It is out there were one could begin to detect some CA.

*Now you are welcome to suggest I also should get my head examined

I retested the Sierra. To me, sharpest in the center of all my binoculars, and also the cheapest. Unfortunately the sharp area is not as wide as in the rest. Anyway, I can not detect the effect in them. Maybe there is astigmatism in the Granite that fits right in with my own if I have any. My eyesight has been tested annually and is well above average (110% I think was the figure given) slightly less on the right eye. But I am not tested in depth for astigmatism. I will now book a full test.

8130rm,

Alas, there are many variables that might be having some effect on all this. Alexis is right about vignetting. I decided not to add that complication in my first reply, but natural side to side eyeball rotation tends to cause more off-axis vignetting in binoculars than up and down eyeball rotation and that tends to clean up the binocular's off-axis performance toward 3 and 9 o'clock compared to 12 and 6 o'clock creating an impression of asymmetrical sharpness even without astigmatism. And both vignetting and astigmatism can vary with light levels as the eye's pupil changes in diameter. I thought astigmatism was the more likely problem here because of the lower sharpness you reported in the field center, which makes sense if there is uncorrected eyesight astigmatism there. Your Sierra could have on-axis astigmatism as a defect, which might be accidentally oriented in way that tends to correct your eyesight astigmatism (if you actually have any).

Another test you might try is to use only one eye at a time. Rotate the binocular in front of the eye and note whether the areas of best sharpness remain at 3 and 9 o'clock or rotate with the binocular optics.

Henry

613Orm said:

I have noticed when looking through my binoculars, and I have a bunch, that the sweet spot, to my view at least, is, to begin with, not symmetrical around the optical axis; it is much wider horizontally than vertically.

Click to expand...

That is normal, in my opinion. Have you tried looking through only one barrel at a time to see if the effect disappears?

The rest of the effect you mentioned could indicate a vision problem. But it is not uncommon that the MTF actually increases somewhat off-center, at least with camera lenses. Image search for "mtf curve", and you will see examples where the MTF increases off-center.