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Elliptical sharpness area of wide-angle binoculars (1 Viewer)

Jessie-66

Germany
Crosspost by Cloudy Nights forum without satisfactory result - but interesting tests and theories:

Hello friends,

I currently own 2 wide angle binoculars with apparent field of view (AFoV) of about 60 degrees (according to ISO 14132-1:2002) or about 66 degrees using the simple formula (TFoV x magnification).

When I look to the edge of the field of view, the sharp area is elliptical (lying ellipse). I see the elliptical sharpness with normal alignment of binoculars - like every person hand holds binoculars. If I turn the binoculars by 90 degrees and look through 1 tube with one eye, I see the sharp area of the field of view circular. This effect occurs with both binoculars, both tubes and both eyes. I therefore exclude eye problems and errors of the binoculars and assume that this is due to the imperfect alignment of the binoculars with eye at a pupil position that deviates more from the optical axis (view to the edge with wide angle binoculars). When looking into binoculars rotated by 90 degrees, you unconsciously align optical axis from the used tube probably better to the pupil with view to the edge - at least differently to normal viewing with 2 tubes to edges/rim. Sorry, english language isn't my native language.

Does this effect also occur with you? Which AFoV has binoculars? Is there a better explanation than I suspect? Is there a technical term for "lying ellipse sharpness area"?

Best regards
Jessie
 

Jessie-66

Germany
Hello,
based on your tip, I experimented:
Normally I adjust the distance between the tubes (IPD) so that I do not see any shadows (kidney beans), optimization also to the effect that a slight offset of the binoculars to my eyes is possible without shadows.
Test:
First I aligned the kink bridge so that the described effect occurs. Then I intentionally set the IPD a little too wide and also (later) a little too narrow (2 different tests).
At the slightly narrower setting (slightly below my optimum to minimize shadowing), I see the sharp area circular. The edge sharpness at the top and bottom increases, the edge sharpness at the left and right decreases, a circular area of sharpness is created.
So I should optimize the setting of the IPD according to another/further/more criterions.
For comprehensibility by third parties:
My IPD is about 62 mm, I observe without glasses. Test with 2 binoculars.
Thanks Maljunolo, Problem solved.

One more conclusion/recognition:
For a halfway objective, public assessment of the sharp area (percent regarding radius of field of view) the reviewer should also take a look with 1 eye into 1 tube of binoculars rotated by 90 degrees. And change eyes and tubes. In this way, assessment errors due to individual circumstances of the reviewer (physiognomy versus eye cups etc.) are at least reduced.
 
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