Why no 'rolling shutter' effect on still images?
This may be a dumb question but I was wondering why we don't see the crazy rolling shutter effects on still images. How's the sensor handling the capture differently between video and stills?
Confused but not entirely perplexed...
not purely a sensor shutter for stills and we normally don't swing the camera rapidly while taking a shot I'm sure helps
sometimes you can see other odd things like baseball bats really, REALLY appearing to flex
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.
The effect is there. Often less pronounced because of different timings.
And anything that would be effected is normally motion blurred, or else the scene looks so surrealistic it doesn't stand out as wrong.
It takes extremely high speeds to see the distortion associated with the subject moving as the shutter curtains move across the frame, but the physics are still there.
This from wikipedia, demonstrating the effect with a modern FP shutter
The normal sequence is shutter opens, image recorded, shutter closes, image read out one line at a time. Once the shutter is closed, the image no longer changes.
In a movie mode, shutter opens, image is being recording, image is read out while image is still being recorded. So, as the image is being read out it's still being changed because the sensor has light on it and there is no electrical way to turn off the light gathering process.
Let's say you can read out the image in 1/200th of a second. With a 1/10th second exposure the read time is only 10% of the exposure time. But with a 1/200th exposure the read time is equal to the exposure time, not good.
Anyone who knows more can correct me, that's what I'm thinking.
|Douaumont Ossuary by Eric 54-BNF|
from Armistice Day
|Silhouette at sunset by Jill Hancock|
from Portrait Lens (around 80mm or equivalent - please check the full rules)