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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?

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It's an interesting question from a physical point of view; it comes down to finding the distance between the curtains and the distance from the curtains to the sensor. But you would have to be photographing one heck of a bright point source for the diffraction to be noticeable at those shutter speeds! Maybe you should experiment with some direct shots into (a reflected) sun... . –  whuber Jan 31 '12 at 22:16

3 Answers 3

up vote 16 down vote accepted

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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While diffraction would occur along the edge of the shutter, the distance to the sensor is so small from the back side of the focal-plane shutter that it would be unlikely to cause any noticeable diffraction.

Remember that diffraction over the area of more than 1 sensor pixel only occurs because there is sufficient distance for the wavelets produced by a tiny aperture in the lens to expand outward before hitting the sensor.

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Saw a discussion on this elsewhere. One expert said that diffraction with 1/8000 would correspond to f/11, which could already be clearly visible with certain cameras. 1/4000 and f/5 would not be.

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Does the answer say the aperture has changed? Or is he implying the effect of the size of the slit between the two shutter curtains is equivalent to diffraction caused by f/11 at 1/8000 and equivalent to diffraction caused by f/5 when used at 1/4000? And that with most APS-C sized cameras diffraction is detectable at f/11 but not at f/5, so it would also be detectable at 1/8000 (even if a wider Av is used) because of the size of the shutter slit but not at 1/4000? –  Michael Clark Jan 1 at 6:12
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I think it would vary from camera to camera since the distance between curtains varies for the same Tv based on how fast the curtains traverse the sensor, and the exact distance from the curtains to the sensor could also vary significantly. Assuming both cameras use vertical plane shutters over sensors the same size, a camera with faster curtains (which should also allow a faster X-sync speed) would use a wider slit for the same Tv as the camera with slower shutter curtains. –  Michael Clark Jan 1 at 6:17
    
@MichaelClark, okay, I see that reading too. Stan, could you explain in a little more detail so it's clear? Some citations would be nice! –  mattdm Jan 1 at 16:47

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