Does focal length affect diffraction, in addition to aperture?

Question

The reason I ask is that f/18 on a 24mm lens = 1.5mm, and f/18 on a 180mm lens = 10mm. I thought diffraction is due to the small physical size of the aperture, rather than the f-ratio, yet I only ever see mention of the f-ratio in discussions of diffraction.

(The lens and camera in my case are an APS-C Nikon D300s and a Sigma 105mm f/2.8, which goes to f64.)

Answer

Excellent question. It boils down to the nature of F-number, which is focalLength/physicalAperture, and the fact that longer focal lengths magnify more. Keep in mind that light projected through an aperture still has to travel from the aperture to the sensor. The greater the distance from aperture to sensor, the greater the magnification...including magnification of the airy disc. The difference between a 180mm lens and a 24mm lens is about 7.5x. To get the same amount of diffraction from a 180mm lens as you would from 24mm lens at f/18, the 180mm lens would need a physical aperture of about 11.25mm in diameter. Given that 180/18 = 10mm, the amount of diffraction present at the sensor is actually a little bit more than with the 24mm lens.

Regarding the Sigma 105/2.8 lens you mention. I believe that is a macro lens. When it comes to macro photography, things change a little bit. You tend to focus extremely close to your subjects with macro photography, so close that depth of field is incredibly small...sometimes millimeters thick. In such situations, it is often more desirable to deal with some diffraction softening as a trade-off for increasing depth of field. In other words, you trade perfect sharpness at the focal plane for additional sharpness beyond the focal plane. Apertures of f/32 or even f/64 are sometimes necessary to even get a shot at all when involving extension tubes.

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