Diffraction Limit

Started Aug 28, 2013 | Discussions thread
Detail Man
Detail Man Forum Pro • Posts: 16,688
Re: The plain truth can indeed be phrased in very misleading ways ...

Paul De Bra wrote:

Great Bustard wrote:

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.

Not the size of the pixels, but the size of the Airy Disk as a proportion of the sensor size. Smaller pixels, for a given sensor size, always resolve more detail at any given f-ratio.

That's only true as long as diffraction isn't spoiling things big time. More pixels on the same sensor size resolve more detail until the airy disk becomes so large that the number of pixels no longer matters. We see resolution curves for different megapixels on the same sensor size converge to the same numbers for the same f-stop when the f-number becomes large enough. So a 24MP full frame sensor will clearly outresolve a 12MP full frame sensor at f/11, it will be close at f/22 and very close to zero difference at f/32. When the airy disk is large enough adding more megapixels to the same sensor surface isn't helping any more. Fortunately for us that is typically at the very end of what a lens can be set to.

True, and this is why the "more pixels always means more detail" dissolves into irrelevant obscurity (in terms of the actual magnitude of "more"), when the extinction frequency of the (lens-system's) MTF cuts into the uppper spatial frequencies to the point where the ratio of "increased detail" asymptotically approaches unity (that is, insignificant amounts of difference).

My calculation in this post find that lens-system diffraction "extinction" (may) become a significant issue at around F=22 (for the E-M5), and around F=27 for the D800:


The largest "un-knownable" in the mix (in constructing such models) is the effective (and not necessarily itself linear) spatial frequency response of the de-mosaicing algorithm utilized.

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