Do very short shutter speeds result in diffraction?

Question

I'm curious if very short exposure times (say 1/8000 or even 1/16000) would cause noticeable blur due to diffraction.

To achieve very fast shutter speeds focal-plane shutters start to close the second curtain before the first one has passed completely over the sensor.

(Illustration from wikipedia)

Is the slit between the front and the rear curtain small enough to make a noticeable impact on the image due to diffraction?

Answer

Slits don't diffract; edges do. There will always be some small amount of the image exposure that arises as the result of diffraction, whether that be from a focal plane shutter or from a leaf shutter. The questions, then, are: how much of a contribution to the overall exposure does diffracted light make; and is there enough angular displacement for that diffraction to matter?

On an APS-C format camera with a 16mm x 24mm sensor and a vertically-travelling focal plane shutter whose curtains traverse the sensor in 1/250s (yielding an expected x-sync speed of 1/200s, allowing for flash duration), when the shutter speed is set to 1/8000s, the minimum gap between curtains will be 0.5mm, which is relatively enormous compared to the wavelengths of the light passing between the curtains. There will be some diffraction, of course, but the degree of interference over most of the slit width will be negligible. The "clear" exposure, the area over which the effects of reinforcement and cancellation have an insignificant effect on the overall magnitude of the incident light, will significantly outweigh the diffraction fringes around the edges of the curtains.

Focal plane shutters, too, are called that because they are very near the focal plane. There isn't a whole lot of room between the shutter curtains and the sensor (or film). The areas of the diffracted light that have significant reinforcement will not be displaced laterally very far, given that they don't have a lot of room to spread out and get comfortable. The distance between sensels on the sensor is much smaller than the width of the shutter slit, being around 7 microns these days, but that is still large relative to the wavelength of light -- light would need to spread out quite a bit before the first few bands of reinforced light (the ones having enough amplitude to affect the overall exposure) started to impinge significantly on neighboring sensels.

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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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