So, what are the m4/3 diffraction limits?

Started Feb 19, 2014 | Questions thread
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bobn2 Forum Pro • Posts: 41,890
Re: So, what are the m4/3 diffraction limits?

pede59 wrote:

bobn2 wrote:

Bob Topp wrote:

As was pointed out above, Cambridge in Colour has a tool for calculating diffraction limits, but that same link provides you an excellent description of the physics:

Bob Topp

The link does not give an 'excellent description of the physics' - it gives a confused, confusing and in some cases downright wrong description. That one site seems to be the source of more myths and misunderstandings than almost any other, and the 'diffraction limited' myth is one of the most widespread and confusing.

It's OK until it says 'When the diameter of the airy disk's central peak becomes large relative to the pixel size in the camera (or maximum tolerable circle of confusion), it begins to have a visual impact on the image.' This is in fact a nonsense. The pixel size does not affect at all the point at which diffraction has a visual effect. The reason for this is that the effects of diffraction and other blurring factors (such as pixel size) do not mask each other, they multiply together. So, this basic misunderstanding carries through the whole piece and everything in this page from there on is basically nonsense, down to the the calculator itself, which might look smart but is in fact completely useless for anything.

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Bob - not sure about the article in reference, but pixel pitch should affect the point at which diffraction has a visual effect provided we have a very good lens (i.e. does not limit resolution of the sensor).

That is the first problem with your thinking. The lens does not 'limit' the resolution of the sensor, nor does the sensor 'limit' the resolution of the lens - the final resolution that you get is a combination of both, which is why it's still worth fitting a good lens to a low pixel count camera or conversely putting a high pixel count camera behind a just OK lens.

Or how else should we interpret MTF charts like the one of the 75mm/1.8? You will see that resolution will drop between f5.6 and f8 already (diffraction limit). If you would use that lens on a sensor with smaller pixel pitch the diffraction limit will be at a higher f-stop (or larger airy disk diameter). E.g. on APS-C/18MP as i recall the drop is between f8 and 11. Or is there something else i am missing?

Yes, you are missing something, which is that using a sensor with a finer pixel pitch does not change the f-number at which the resolution starts to drop due to diffraction. It changes the resolution that you get, but the drop starts at exactly the same f-number. Here for example is the MTF50 of a Nikon lens on two different Nikon cameras with different pixel pitch:

Notice that in both the resolution falls away after f/5.6. The 24MP camera extracts more resolution from the lens than the 12MP one.

Notice also there is no defined 'limit' where the resolution suddenly falls due to diffraction, it is a smooth and even drop-off. The 'limit' is just a bogus idea. McHugh has taken a well defined optical term - a 'diffraction limited' system is one so good that diffraction is the only limit on its performance - turned it inside out and made it into something senseless.

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