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05-30-2012, 07:46 PM   #91
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The background idea is that finer pixels show the negative effect of diffraction at wider apertures than do coarser pixels.
This was demonstrated very well by the link
Diffraction Limited Photography: Pixel Size, Aperture and Airy Disks
that Bossa posted.

Now think of diffraction as making a cone emanating from the lens,
where there should just be a single ray.
If that cone is at the edge of the image,
so the sensor is cutting the cone at an angle,
the cut will be a bigger ellipse, not just a circle.
So the ellipse will cover more pixels than the circle would,
and Imatest, counting pixels,
would indicate more diffraction degradation at the edge than at the center.

Don't know if this helps to clarify the idea,
within a semi-casual internet forum, not an optics seminar!
I'm just brainstorming a potential solution to Klaus' puzzle.

05-30-2012, 07:58 PM   #92
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QuoteOriginally posted by lytrytyr Quote
The background idea is that finer pixels show the negative effect of diffraction at wider apertures than do coarser pixels.
This was demonstrated very well by the link
Diffraction Limited Photography: Pixel Size, Aperture and Airy Disks
that Bossa posted.

Now think of diffraction as making a cone emanating from the lens,
where there should just be a single ray.
If that cone is at the edge of the image,
so the sensor is cutting the cone at an angle,
the cut will be a bigger ellipse, not just a circle.
So the ellipse will cover more pixels than the circle would,
and Imatest, counting pixels,
would indicate more diffraction degradation at the edge than at the center.

Don't know if this helps to clarify the idea,
within a semi-casual internet forum, not an optics seminar!
I'm just brainstorming a potential solution to Klaus' puzzle.
I read the article, and unfortunately it really glosses over the airy disk to pixel relationship. The overall airy disk of a plane wave entering any given aperture (as shown in the background section of the article) is different than the airy disk formed by a fine detail. I don't mean different in terms of physics, but different in terms of the amount of coverage. The plane wave example shows what will happen overall to the diffraction limit of the lens (ie, the MTF drops in the higher frequencies). The airy disk of a detail (or the pair of airy disks as shown in the Rayleigh model) is a much smaller diameter and is spatially only a portion of the rays entering the system.

ETA...I understand now that you are talking about at what aperture the diffraction limit will occur...
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