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In answer to another question, Adam Davis writes:

Your camera complicates this by using a rolling shutter above a given speed (usually around 1/200. This means that only a portion of the image sensor is exposed to the scene at any given time, so if the light changes during the exposure, the color change will only affect a portion of the image sensor.

Rolling shutters often are also mentioned in the context of DSLR videography. However, I am yet to see a discussion of what a rolling shutter is, how it works and when it is important.

What is a rolling shutter?

What are the implications of using one for my photos?

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What Adam is talking about is not actually a rolling shutter, it's just a focal plane shutter. It also does nothing special above 1/200. –  thomasrutter Mar 7 '11 at 10:36
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4 Answers

up vote 12 down vote accepted

What Adam is referring to

What Adam is talking about is not actually a rolling shutter, it's just a focal plane shutter. It also does nothing special above 1/200, except that the effect of the shutter curtain has some interesting properties which can become more pronounced at higher speed.

The diagrams on the wikipedia page (reproduced below) illustrate it best. Essentially the shutter consists of two curtains which move from top to bottom (or in some film cameras, left to right) in quick succession. The gap between them is what exposes the image.

Focal plane shutter, low speed

Focal-plane shutter, low speed. Black square is the sensor, red and green squares are the first and second curtains.

Focal plane shutter, high speed

Focal-plane shutter, high speed. Black square is the sensor, red and green squares are the first and second curtains.

If the shutter speed is fast enough, the second one will start closing before the first one has fully finished opening, so the entire frame won't all be exposed at once. Therefore, you get a situation where anything that happens really fast, like the flash of a camera or the oscillation of a fluorescent light, may cause light not to cover the entire frame but instead create bands or gradients from top to bottom where the light is different.

What a rolling shutter effect is

The rolling shutter effect as it applies to digital video is quite a different and quite unrelated effect to the one described above. Actually, a rolling shutter effect does not actually involve a physical shutter, but it's called that as a convention because it is analogous to the way a film cinema camera has a shutter that moves across the frame. In digital video, the rolling shutter effect is the result of the way a CMOS sensor is read.

CMOS sensors exhibit a rolling shutter effect when they are in live view or video mode, in which they are being read for every video frame. Instead of capturing the entire frame at once, information is read from each row of the frame one after the other, top to bottom. The whole process takes up to 1/30 of a second on most cameras. This creates a jelly-like wobbling effect in recorded video when the camera is handheld or moves a lot.

In a given sensor, this rolling shutter happens equally regardless of the shutter speed, though with slower shutter speeds it may be less noticeable in subject movement because of the extra motion blur. The effect is not usually noticeable when the camera is fixed on a tripod or panned steadily, but is more obvious when the camera is hand-held or during fast camera movements.

CMOS sensors capable of higher frame rates than 30 frames per second (and not just through repeating frames) will exhibit less rolling shutter effect because their sensors will have been designed to be read faster.

CCD does not suffer from the rolling shutter effect.

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+1 Great explanation, thank you. I've inserted the referenced images from wikipedia, I hope you don't mind. –  fmark Mar 8 '11 at 22:42
    
Yep that's good, thanks. In case people are wondering, the diagrams show the shutters moving horizontally whereas modern cameras seem to have vertical shutters. Same effect though. –  thomasrutter Mar 9 '11 at 2:04
    
I made a similar question and was down-voted, because this explanation does not show up in the search. Thanks for the explanation on matrices. –  culebrón Mar 10 '13 at 19:24
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Adam's terminology is slightly out - you don't get a rolling shutter above 1/200s or so, but one whose where the bottom edge starts to close before the top edge has fully opened, but the effect is the same (a rolling shutter implies the scene is constantly read out from top to bottom).

Effectively what you get at high speeds is a slit that moves up the sensor exposing one part at a time. This has implications for large changes in lighting during the exposure, such as flash, or in the original context florescent lights which vary during the AC cycle.

Lots of related questions for further reading:

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No, a rolling shutter doesn't imply that the image is read repeatedly. –  Guffa Mar 7 '11 at 15:13
    
Doesn't it? see my comment on your answer! –  Matt Grum Mar 7 '11 at 16:46
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The shutter is actually two shutters, one that slides up and one that slides down. One is used to start the exposure and the other one is used to end the exposure.

It takes a little bit of time to open a shutter, so if there was only one shutter there would be a difference in exposure time between the top and the bottom of the image. If for example the shutter needs 1/1000 s. to open, that would mean a 20% difference in exposure time between the top and the bottom when exposed at 1/100 s.

The term rolling shutter is used when the second shutter starts to close before the first shutter has finished opening. In the extreme case this means that there is just a thin opening that rolls across the film/sensor. You could get the same function from a single shutter in the form of a curtain with an opening that rolls across the media, but that construction would give you less flexibility when it comes to exposure times.

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I would disagree on the terminology, rolling implies something is constantly happening in a cycle, such as a rolling roster or rolling substitutes in sport, and as such only applies to repeatedly reading the image line by line electronically as in live view, not to the operation of a mechanical shutter. –  Matt Grum Mar 7 '11 at 10:07
    
@Matt Grum: This, for example, disagrees with you: en.wikipedia.org/wiki/Rolling_shutter –  Guffa Mar 7 '11 at 15:09
    
That's Wikipedia for you, the article seems to contradict itself on the subject: "This method is implemented by rolling (moving) the shutter across the exposable image area instead of exposing the image area all at the same time (the shutter could be either mechanical or electronic). The advantage of this method is that the image sensor can continue to gather photons during the acquisition process, thus increasing sensitivity." How can the image sensor continue to gather photons using a mechanical rolling shutter? –  Matt Grum Mar 7 '11 at 16:43
    
n.b. I didn't downvote you, the rest of your answer contains useful info, despite a difference of opinion of the definition of rolling... –  Matt Grum Mar 7 '11 at 16:59
    
Wikipedia likes to contradict itself sometimes. See also the article on the focal plane shutter which at no point refers to it as a "rolling shutter" en.wikipedia.org/wiki/Focal-plane_shutter - amusingly they even include the same helicopter image as an example of a focal plane shutter effect. I think, and this is what I personally feel, that it's a common misconception that that is what a "rolling shutter" is. –  thomasrutter Mar 8 '11 at 0:14
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This movie on youtube is the best explanation of this I've seen.

I discovered it on this page on DYI photography site

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