What really is the deal with diffraction?

Started Apr 22, 2014 | Discussions thread
hjulenissen Senior Member • Posts: 2,106
Re: What really is the deal with diffraction?

aftab wrote:

1. What is diffraction limit? Is it when two adjacent Airy discs start to overlap each other? Is it when one Airy disc begin to cover two or more adjacent pixels? Both?

When talking about "the diffraction limit", it seems that people generally are talking about the Rayleigh criterion. In laymans terms, the distance where one airy disc just has its first minima at the maxima of another airy disc. This seems to be (related to) an empirical limit on how human observers are able to visually distinguish two stars, depending on the blur that is caused by diffraction in optical telescopes (se figure and link below)

While relevant as a rule of thumb, I think that (some) enthusiasts tends to treat this as a fundamental physical limit, something that it is (as far as I know) not. I.e. ">24 megapixels will give you exactly zero added information compared to 24 MP at apertures of f/some-number and below due to the diffraction limit". There have been discussions of such an absolute limit on this site, but I believe that numbers were quoted into gigapixel region for relevant sensor sizes and apertures, for worst/best-case wavelengths and color filter subsampling.

What is more important to me is:

0. Most interesting scenes are not flat brick-walls. Thus, appearance outside of the plane of focus is often relevant (and affected by aperture).

1. There are many sources of blur, of which diffraction is but one.

2. It is actually possible to "repair" blur to some degree, but this tends to be time and cycle-consuming, and rely on accurate models of the blur, low noise, (possibly scene) etc.


I am sure that we will (practically) bang our heads up against the physical limits sooner or later, but for now it seems worthwhile to optimize what we have for those applications where improved quality is warranted. I.e. when 54MP FF cameras are available, or Fossums JOT-sensor is available, or some other asymptotic approach to the ideal sensor (lens limited system), I am sure that some people will be able to turn that technology into images that would have been hard(er) to achieve using todays tech.


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