Diffraction Limit

Started Aug 28, 2013 | Discussions thread
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Great Bustard Forum Pro • Posts: 33,065
Diffraction in terms of pixel count, DOF, and sensor size

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.

Indeed.  In the penultimate paragraph of the quoted section in my post, you may have read the following:

In other words, all systems will suffer the same amount of diffraction softening at the same DOF and display dimensions. However, the system that began with more resolution will always retain more resolution, but that resolution will asymptotically vanish as the DOF deepens.

In addition, in another post above, from the same link as the above quote:

However, the relationship between diffraction softening and pixel density is largely misunderstood. For a given sensor size and lens, more pixels always result in more detail. As we stop down and the DOF deepens, we reach a point where we begin to lose detail due to diffraction softening. As a consequence, photos made with more pixels will begin to lose their detail advantage earlier and quicker than images made with fewer pixels, but they will always retain more detail. Eventually, the additional detail afforded by the extra pixels becomes trivial (most certainly by f/32 on FF). See here for an excellent example of the effect of pixel size on diffraction softening.

In addition, from yet another post above:

  • There is no such thing as a "diffraction limit" except when the resolution falls to zero.
  • There is a point where diffraction softening becomes the dominant source of blur, and this point will vary from lens to lens, as well as where in the frame we are looking (the corners typically, but not always, lag about a stop behind the center for DSLR lenses).
  • All systems suffer the same diffraction softening at the same DOF.
  • More pixels, all else equal, will *always* resolve more detail.
  • All systems do not necessarily resolve equally at the same DOF, as diffraction is one of many sources of blur. However, as the DOF deepens, the resolution decreases, and the resolution differences between systems narrows, typically becoming trivial by f/16 on mFT (f/32 on FF and f/5.6 on an FZ200), regardless of how sharp the lens is or how many pixels the sensor has.

Gotta love copy and paste.

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