Why no 'rolling shutter' effect on still images?

Started Oct 5, 2009 | Discussions thread
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Cutty Junior Member • Posts: 46
Re: Why no 'rolling shutter' effect on still images?

My understanding (which may be entirely incorrect!):

Rolling Shutter/Video Mode:

Because (1) it takes a certain amount of time to read exposure data and (2) sensors are constantly accumulating charge when exposed to light, when shooting video it's necessary to traverse the image row by row, reading out the exposure values and resetting the accumulated exposure back to zero. There's enough delay that by the time the camera is reading out the last row of data, it has effectively exposed something "later in time" than the first row. When there's a lot of changing visual information (panning camera or fast in-frame movement) this can cause distortions in the image.

Mechanical Shutter/Still Mode:

Prior to exposure, the entire sensor is reset. The shutter is then tripped, exposing the entire sensor to light for a period of time, during which the sensor accumulates charge from the light. Once the shutter is closed, the camera reads out the data from the sensor sequentially just as with video, but unlike video the exposure is not happening simultaneously with read-and-reset, so there are no distortion artifacts caused by the sequential read.

...

(Warning: If the above isn't entirely clear, what follows will just cause more confusion!)

I suppose in theory with a very fast exposure and a lot of movement (camera or in-frame) it should be possible to see a similar distortion in stills due to the mechanical shutter being comprised of two "curtains". With very fast exposures the second curtain is already closing before the first curtain is finished opening, causing a strip of light to scan across the sensor.

In practice, you'd need a lot of light and something moving very fast to see this, as otherwise the artifact would be lost in motion blur. But in this case the same artifact would appear on film since the physics of the exposure are effectively the same.

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