Started 7 months ago | Discussions thread
fvdbergh2501 Contributing Member • Posts: 516
Re: BxU & Diffraction

alanr0 wrote:

fvdbergh2501 wrote:

hjulenissen wrote:

It seems to me that "micro contrast" is a term like "pop" or "3d-ness" that can mean pretty much anything.

Did DXO release a paper at one time where they found that the subjective quantity "sharpness" was better correlated with the _derivative_ of MTF at zero frequency (tilting) rather than MTF50 or MTF10?

Looking at the BxU paper that DetailMan found for us, it looks like they are using the second derivative of MTF at zero frequency (Section 5.1, "Estimating the blur"). This will work as expected if the MTF is Gaussian (which seems to be the theme of the paper overall), i.e., <snip>

As you say, their thesis seems to be

  • A Gaussian point spread function has convenient mathematical properties.
  • If the PSF is Gaussian, so is the MTF.
  • If the PSF is Gaussian, then its second moment is a useful measure of blur, and is proportional to the second derivative of the MTF at the origin.

I do not believe your diffraction-limited and spherical aberration curves satisfy Buzzi & Guichard's conditions for extensivity and additivity.

As far as I can tell, an Airy disk point spread function is not contained within the Schwartz space as the higher order derivatives do not decrease sufficiently rapidly.

Their blur measure is essentially the second moment of the point spread function. For an Airy disk (and also for a diffraction-limited square aperture sinc^2 PSF) the second moment integral is ill-defined, increasing indefinitely as the size of the integration region increases.

Thanks Alan.

As they say, by putting in two days of hard work in the lab, you can avoid having to spend 15 minutes in the library

I wonder what they intended to measure with this blur metric if it does not allow Airy-like PSFs?

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