Why do DSLRs in movie mode have a rolling shutter instead of a global shutter?

Question

Correct me if I'm wrong:

When I take a picture, the sensor gathers the light information "as a whole" during the exposure time and then saves it in the card. When I make a movie, the sensor scans line by line and then saves the frame in the card.

So, if this is correct, why does the camera switch from global shutter (as in photo mode) to rolling shutter (movie mode) when filming?

Of course there must be a technical reason, but why bother using a rolling shutter instead of a global shutter, which is much more convenient?

Answer 1

In both cases, the actual reading from the sensor is done line by line. It's done that way (largely) because doing otherwise would be excessively expensive -- to read all the pixels in parallel, you'd need a separate connection from the sensor to memory for each pixel. 12 million connections (for example) from sensor to memory would be horrendously expensive -- and almost never provide any real benefit.

As to why there appears to be a difference between movie mode and still mode, it's pretty simple: in still mode, you're using a physical shutter, and the read-out from the sensor to memory happens when the shutter is closed. As @Matt Grum pointed out, you still get some of the same effect above the X-sync speed, due to physical limitations on the shutter.

The reason you don't use the physical shutter in movie mode is more of those same physical limitations. While the shutter can have a very short exposure time, there's a recovery time between activations, so it becomes difficult to achieve more than about 10 frames per second or so. Getting to the 24 frames per second or so needed for video would, again, add a lot of expense with little benefit. Therefore, in movie mode the physical shutter remains open, and the camera uses an electronic shutter instead -- and once it does that, artifacts of the line-by-line readout from the sensor to memory can become visible.

Answer 2

When taking a still you have a physical shutter which controls light hitting the sensor. The image is still read line by line when taking stills but because of the physical shutter closing no extra light is recorded during readout.

The line by line readout is a consequence of the CMOS design found in large sensor cameras, and thus is unavoidable (until they find a way to make cheaper/bigger CCDs).

It's worth noting that at shutter speeds faster than the cameras x-sync speed (usually 1/250s) the shutter starts closing at the bottom before it's fully open at the top. The result of this is that for really fast speeds like 1/4000s you get a slit that traversed the frame and gives you a similar rolling shutter effect for stills. However because the time to traverse the frame by the shutter (1/250s) is ten times faster than the time to read the sensor during video (1/25s) you need a really fast moving object to notice it.

Here's a very old photograph which demonstrates the effect well:

The effect is also more noticable in videos if you pan back and forth, which doesn't happen with stills.

Answer 3

You may find this link useful:

http://www.dvxuser.com/jason/CMOS-CCD/ (by Barry Green /via@SFGPhoto)

It's a CMOS compromise: heat, power, slower refresh rate, cheaper than CCD. CCDs are faster, but need more energy and cost more...

From my basic understanding of chip manufacturing, image sensors and processors are created using similar tech. Processor fabrication favours shrinking: smaller = cheaper. But photographers want BIG sensors. Can't have it both ways without it costing a lot more. Plus packing in lots of pixels into a sensor can be too much of a 'good' thing (less light etc.)

@mogwailun