Diffraction Limit

Started Aug 28, 2013 | Discussions thread
Paul De Bra
Paul De Bra Forum Pro • Posts: 12,513
The plain truth can indeed be phrased in very misleading ways...

LTZ470 wrote:

From a very credible source:

"Diffraction thus sets a fundamental resolution limit that is independent of the number of megapixels, or the size of the film format. It depends only on the f-number of your lens, and on the wavelength of light being imaged. "

This statement is absolutely factually correct. It is also a useless statement for anyone trying to decide which higher f-numbers to avoid to not suffer from softening due to diffraction.

Diffraction indeed defines how light is "scattered" over an area of the sensor, depending on the f-number. The size of the "airy disk" is independent of the number of megapixels or the size of the film format. But, now let's see whether that has influence on the image we can capture:

Suppose the diameter of the airy disk is X. If the size of our sensor is of the same order of magnitude as X the total image captured will have absolutely no detail whatsoever, even if that tiny X-sized sensor has 100MP. If the size of our sensor is in the order of 1000X we can resolve in the order of 1000 lines and get a fairly detailed image. The size of the airy disk is the same but on a larger sensor the *relative* size of that airy disk to the total size of the image is way smaller.

So what matters is the size of the airy disk that corresponds to a certain f-stop versus the size of the pixels relative to that airy disk. When you look at pixel-level detail of an image (which is not overall sharpness) what counts is the size of the pixels. Diffraction at the pixel level at say f/16 on a 12MP 4/3 sensor will be roughly equivalent to diffraction at f/16 on a 48MP 35mm full frame sensor, when you look at the image from the same distance relative to the pixel size. So if you look at a 30x20 print from that 12MP sensor from 2 feet you should look at a 60x40 print form the 48MP sensor from the same 2 feet.

Diffraction does play a different role on smaller versus larger sensors because the way I described above is not the way we look at pictures. We blow up the pictures captured by small sensors and/or shrink the pictures captured by large sensors so that the printed images are of the same size. When you do that we are effectively changing the effect the size of the airy disk has. We are blowing up the airy disk seen by the small sensor and/or shrink the airy disk seen by the larger sensor. Which means that the diffraction caused by a certain f-stop becomes much more visible in the blown-up image from the small sensor and less visible in the shrunk image from the larger sensor.

So while the "fundamental" statement is correct the reality is that smaller sensors suffer from diffraction at larger apertures than large sensors because we change the magnification of the image captured by the surface of the sensor.

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