07-07-2014, 09:19 PM
>Onion ring bokeh is just a sign that an aspherical element is not totally smooth
That's the jist of it, but the lack of smoothness results in hot and cold spots across the lens' surface. Which is a lack of acuity. Which is an alteration of the point spread function from ideal... in a concentric manner...which is sagittal... which is astigmatism.
It's less "not totally smooth" and more "not ground finely enough - visible peaks at detectable scales."
>Could you please expand on this thesis
There are several issues which will alter the PSF of the lens. Astigmatism will alter it - if tangential resolution is lower than sagittal you will have spikes that form concentric 'wings' - if sagittal is lower, you get spikes that point towards the optical axis.
Mechanical vignetting will alter it outside the center of the lens by shaping it to reflect the apparent iris.
Pinched optics will alter it by causing astigmatism-esque changes to the PSF, but the changes will not be symmetric. Standard QC results in lenses being centered, so it is highly unlikely that an element was not centered before it even became coated, let alone be allowed into a produced lens. When a lens is pinched, the stress alters its centering (please note - centering in this case is slightly different to decentering for a photographic lens, which is a system of lenses. In this case, it means the center of curvature matches the physical center, whereas for an optical system it's a matter of alignment) which will produce asymetrical astigmatism.
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> And pinched optics?? Really? No.
You're welcome to guess what the problem is, just as I am. But please try not to be so dismissive of others - not everyone has the same knowledge base, and it's possible that your guess is wrong
Then again, mine could be as well.
That's the jist of it, but the lack of smoothness results in hot and cold spots across the lens' surface. Which is a lack of acuity. Which is an alteration of the point spread function from ideal... in a concentric manner...which is sagittal... which is astigmatism.
It's less "not totally smooth" and more "not ground finely enough - visible peaks at detectable scales."
>Could you please expand on this thesis
There are several issues which will alter the PSF of the lens. Astigmatism will alter it - if tangential resolution is lower than sagittal you will have spikes that form concentric 'wings' - if sagittal is lower, you get spikes that point towards the optical axis.
Mechanical vignetting will alter it outside the center of the lens by shaping it to reflect the apparent iris.
Pinched optics will alter it by causing astigmatism-esque changes to the PSF, but the changes will not be symmetric. Standard QC results in lenses being centered, so it is highly unlikely that an element was not centered before it even became coated, let alone be allowed into a produced lens. When a lens is pinched, the stress alters its centering (please note - centering in this case is slightly different to decentering for a photographic lens, which is a system of lenses. In this case, it means the center of curvature matches the physical center, whereas for an optical system it's a matter of alignment) which will produce asymetrical astigmatism.
----
> And pinched optics?? Really? No.
You're welcome to guess what the problem is, just as I am. But please try not to be so dismissive of others - not everyone has the same knowledge base, and it's possible that your guess is wrong
Then again, mine could be as well.