optics - Does sensor size impact the diffraction limit of a lens?

My understanding of diffraction, is that with a small aperture the 'airy disk' (which I understand is the pattern light from a given direction relative to the lens will form on passing through the lens), becomes larger, and thus overlapping of these airy disks occurs. The lens diffraction limit is when two or more of these airy disks overlap on a single photo-site on a sensor or cross over onto two photo-sites, causing reduced sharpness. Therefore, if the sensor is larger, and the photo-sites for the same resolution can also be larger, does this influence the diffraction limit of a lens? if so, how?

Answer

Does sensor size impact the diffraction limit of a lens?

No.

Therefore, if the sensor is larger, and the photo-sites for the same resolution can also be larger, does this influence the diffraction limit of a lens?

Not really. What it does affect is the sensor's (not the lens') diffraction limit.

If so, how?

If the size of the Airy disc caused by diffraction is smaller than the ability of the sensor (or film grain) to resolve it, then the image will not be diffraction limited. Only when the size of the Airy disc is large enough to be resolved by the sensor will the image be diffraction limited. The resolution limit of the sensor is determined by the pixel pitch: that is, the distance of the center of each pixel well from adjacent pixel wells. The aperture at which the sensor can resolve the Airy disc is what we refer to as that sensor's Diffraction Limited Aperture (DLA).

Diffraction Limited Aperture (DLA) is only applicable at 100% viewing size. This is because DLA assumes a Circle of Confusion (CoC) equal to the pixel pitch of a particular sensor. The effects of diffraction at the DLA are only observable if the resulting image is magnified enough that the viewer can discretely resolve individual pixels. For an 18MP image viewed on a 23" HD (1920x1080) monitor that is the equivalent magnification of a 54"x36" print!

Take for example the 20.2MP full frame Canon 6D and compare it to the 20.2MP APS-C 70D. Both have the same resolution: 5472x3648.

• The 6D has a pixel pitch of 6.54µm and DLA of f/10.5


• The 70D has a pixel pitch of 4.1µm and DLA of f/6.6

The lower DLA of the 70D is due to its smaller sensor/pixel size that requires higher magnification to display images from the 70D at the same size as an image from the larger sensored 6D.

Diffraction at the DLA is barely visible when viewed at 100% (1 pixel = 1 pixel) on a display. As sensor pixel density increases, each pixel gets smaller and the DLA gets wider. DLA does not mean that narrower apertures should not be used. It is where image sharpness begins to be compromised for increased DOF. Higher resolution sensors generally continue to deliver more detail well beyond the DLA than lower resolution sensors until the "Diffraction Cutoff Frequency" is reached (a much narrower aperture). The progression from sharp to soft is not an abrupt one. For more about diffraction, read this question. Current Canon DLSRs may have a DLA as low as f/6.6 (70D, 7DII) and as high as f/11 (EOS 1D X). Most other manufacturers' DSLR offerings fall somewhere along the same lines.

Ultimately you must consider all of the factors involved to decide what is the best aperture to use for a particular photograph. Many times, it will be a compromise between several factors such as more depth of field (narrow aperture) and usable shutter speed and ISO (wide aperture).