The whole question of lens sharpness...

Started Jun 12, 2013 | Discussions thread
Alphoid
Senior MemberPosts: 2,472
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Re: Theoretical deconvolution vs practical
In reply to hjulenissen, Jun 22, 2013

hjulenissen wrote:

1. It is impossible to measure the PSF perfectly, and if you did, it might change ever so slightly 2 seconds later

True indeed.

Systems that have an approximate measurement (DxO) do pretty well. Doing a near-exact measurement for a lens would be very expensive.

2. To know all output, you would need the outputs beyond the sensor. I guess that this can be sorted out by discarding results that are 1 or 0.5 effective PSF kernel width from the image edge (cropping), mirror-imaging the image to articficially get something to work on, or living with image artifacts at the edges.

Irrelevant, for the reasons you mention later. The PSF is pretty tiny. We're talking about a tiny, few-pixel crop.

This is a big deal for audio, where you really do end up with long transients.

3. The PSF might be very large. Perhaps a deconvolution + contrast modelling works to give good pictures, but surely there must be cases where the PSF reduce contrast in the right half of the image and not the left half. Global contrast adjustement would not fix that, and a 2MP PSF would be impractical?

For the purposes of sharpness (PSF is more than 3dB or so), the relevant part of the kernel really is quite tiny.

4. The kernel might contain infinitely deep zeros (located on the unit circle of the Z-transform). These represent complete loss of information that cannot be brought back. I don't know if this is common or possible for PSF.

This is very hard to do optically, but not impossible. See figure 4 in:

https://graphics.stanford.edu/courses/cs448a-08-spring/levin-coded-aperture-sig07.pdf

Diffraction aside, you can't practically subtract light (this is different from audio). As a result, the way to get zeros is to add light in really funny patterns.

This is not something I'd expect to see in any real-world lens.

This is something you do see in acoustics, where you will have standing waves in a room and the like.

5. Light itself contains noise (being a stream of photons).

No problem.

6. Any real camera contains an image sensor with a CFA, limited density sensels, clipping highlights

CFA and clipping are potential problems, indeed.

and noise.

Most also have an OLPF. We dont get to process the light from the lense until it has been further distorted by those things.

No problem. I intentionally left this out of the discussion, but this has a PSF as well that could be included in the model. In that case, we'll compensate for the overall blur better than for just the blur of the lens (although, depending on OLPF design, possibly not perfectly). Or we can exclude it, in which case we only correct for blur of the lens, but perfectly.

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