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Is the "sweet spot" wider horizontally than vertically?

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#1 asphericalaberration

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Posted 23 May 2020 - 02:48 PM

In looking critically through five vintage ultra-wide porros, I've noticed that the "sweet spot" seems consistently wider horizontally than vertically. I mean that the top and bottom edges of the image are less sharp than the right and left edges. This effect is visible when viewing with both eyes and also when shutting each eye, so it seems to be a feature of each tube separately. Is sharpness asymmetrical in porros? Or am I just seeing things, as it were? (Lazy me, haven't checked roofs for this effect yet.)


Edited by asphericalaberration, 23 May 2020 - 02:52 PM.

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#2 Mark9473

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Posted 23 May 2020 - 03:13 PM

I think it's your eyes.



#3 asphericalaberration

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Posted 23 May 2020 - 03:36 PM

You're probably right. One-eyed testing with bins in normal orientation versus tilted 90* seems to show the same pattern, i.e., wider sharpness is horizontal in both cases. It's subtle, though, and I don't have a good way to quantify, so it may take a while to be sure. I don't see a reason for optics to be asymmetrical. I wonder if this is "my" vision versus "human" vision (maybe wider horizontal acuity had a selective advantage over time).

 

Also my wife, unaware of the question, found the testing either amusing or concerning, she won't say which.


Edited by asphericalaberration, 23 May 2020 - 03:40 PM.


#4 j.gardavsky

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Posted 23 May 2020 - 03:45 PM

The Porros I have checked with my eyes, have had an asymmetry in the pin point definition up/down vs left/right,

as you describe it.

I have not seen it with my eyes on my Swarovski EL 8.5x42 Swarovision roofs.

 

Best,

JG


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#5 KennyJ

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Posted 23 May 2020 - 03:58 PM

I don't know about anyone else's eyes, or if this bears any relationship to the phenomenon reported, but if I sit as motionless as possible and try to see how close to 180 degrees I can see from the corners of my eyes in a horizontal plane, then try the same from up to down, I'm sure I can see closer to 180 degrees in the horizontal plane.

 

Kenny


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#6 asphericalaberration

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Posted 23 May 2020 - 04:02 PM

On second thoughts, I'm less sure it's my vision. I'm using the sharp center of my vision to stare at the edges near the field stop. Using the same portion of my vision for all tests should exclude vision as a cause. Two pairs of modern roofs don't seem to show this effect. It's subtle, though, so I'm a little uncertain.


Edited by asphericalaberration, 23 May 2020 - 04:05 PM.

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#7 asphericalaberration

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Posted 23 May 2020 - 04:03 PM

>I'm sure I can see closer to 180 degrees in the horizontal plane.

 

Peripheral vision is asymmetrical, wider than tall.



#8 SMark

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Posted 23 May 2020 - 07:22 PM

On second thoughts, I'm less sure it's my vision. I'm using the sharp center of my vision to stare at the edges near the field stop. Using the same portion of my vision for all tests should exclude vision as a cause. Two pairs of modern roofs don't seem to show this effect. It's subtle, though, so I'm a little uncertain.

I see this as well. I tend to think it might an "Extra Wide Angle" phenomenon. 


Edited by SMark, 23 May 2020 - 07:22 PM.


#9 Mark9473

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Posted 24 May 2020 - 04:10 AM

On second thoughts, I'm less sure it's my vision. I'm using the sharp center of my vision to stare at the edges near the field stop. Using the same portion of my vision for all tests should exclude vision as a cause. 

To be certain, you'd have to rotate the binoculars in steps of 90° so that your eye is always in the same position.


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#10 Jon Isaacs

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Posted 24 May 2020 - 05:11 AM

I think it's plausible that porros are asymmetrical in terms of the sweet since the prisms are asymmetric.

 

Roofs, while not exactly symmetric, are most more so.

 

Jon


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#11 garret

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Posted 24 May 2020 - 07:17 AM

In my APM 100ED APO binocular telescope the stars in the top edge are less sharp than the stars in the bottom edge, both tubes.

Left and right edge sharpness are in between top and bottom.

The best edge sharpness is in the seven o'clock position.


Edited by garret, 24 May 2020 - 07:26 AM.


#12 asphericalaberration

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Posted 24 May 2020 - 11:39 AM

I'm now pretty convinced this effect is real. I'd assume it happens in any porro and becomes visible in ultra-wides because they're pushing the limits of the optics. I'd surmise that prisms, not lenses, are the cause, and it likely has to do with the sequence of the prisms. I've believed that one prism handles the vertical flip and the other handles the horizontal flip. The prisms in the porros I've just looked at are mounted at right angles to each other but "diagonally" with respect to the horizon, however, so it's not immediately obvious which one (or if only one) handles the lateral flip. Is it correct to surmise that the first (back) prism is the one closer to its limits in an ultra-wide, i.e., under more "optical stress," and the more likely cause? I'm a bit embarrassed to ask, since I used to think I knew how the darned things worked.

 

Edit: D'oh! There's no "vertical" or "horizontal" in the raw image apart from our interpretation of it. When you look at something and tilt your head, the image doesn't tilt. The fact that the prisms are 90* apart properly inverts the image. The mounting angle of the major axis of a prism with respect to the horizon at any particular moment doesn't matter. Thirty seconds with a pair of loose prisms will prove it.


Edited by asphericalaberration, 24 May 2020 - 01:12 PM.

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#13 Jon Isaacs

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Posted 24 May 2020 - 11:58 AM

I'm a bit embarrassed to ask, since I used to think I knew how the darned things worked.

 

 

I'm not among those who think they know how binoculars work.. I have some idea but if asked to draw a ray diagram there would be a big empty box with a question mark drawn around the prisms..

 

Jon



#14 j.gardavsky

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Posted 24 May 2020 - 12:30 PM

I think it's plausible that porros are asymmetrical in terms of the sweet since the prisms are asymmetric.

 

Roofs, while not exactly symmetric, are most more so.

 

Jon

This is a plausible explanation,

 

JG



#15 Rich V.

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Posted 24 May 2020 - 01:05 PM

With Porros, what we see as up/down and left/right is relative; it depends on the IPD of the user.  That relationship could easily vary by 45° depending on the hinge positon.  I can't think of any logical reason why the rotational position of the prism assy would have any effect on a "sweet spot" of focus between the center and edges unless the prisms are somehow tilted and not aligned square with the principal axis of the objectives/eyepieces.  We commonly see the effects of tilted prisms causing an off-center "sweet spot" on inexpensive binos that may be misaligned but I wouldn't expect it in higher quality models.

 

Illumination can fall off on WA designs because of critical reflective angle and glass type used in the prisms, but not sharpness.  Some WA Porro binos, Nikon EIIs as an example, use two different sized prisms on each side; the larger prism's faces looking at the objective and the smaller at the eyepiece to follow the narrowing light path but not intrude into it.

 

Rich


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