Why do electronic shutters expose row by row?

Question

This question connects with my previous question, Why do cameras use a slit between two curtains rather than exposing the entire sensor instantly? This question is about why the slit thing comes with the electronic shutter.

Why would electronic shutters expose row by row though there a slit is not needed?
Why can't the sensor be exposed for a moment, globally (electronically)? Oh, and it's not exposing, is it? It's more like 'downloading the image to the processor, isn't it?

So, why can't the sensor's image data downloaded to the processor, globally? Why is it downloaded row by row?

Answer 1

So, why can't the sensor's image data downloaded to the processor, globally? Why is it downloaded row by row?

It's a matter of physical limitations and simplicity. The physical limitation is that there's only space for a certain number of external connections -- you couldn't possibly connect every pixel to the processor and grab all that data at once because there's not enough room for the 20 million (or even more) little wires that would have to go from the sensor to the processor. There's not even enough room (on either the sensor or the processor) for enough external connections to read all the pixels in a single row at exactly the same time. So, you need to be able to store the image on the sensor somehow so that you can read it back a little bit at a time.

What you could do is have each pixel connect to its own little memory cell, so that when you trigger the "shutter" every pixel would store its value in its respective memory location at exactly the same time. That's called a global shutter, and some image sensors work exactly that way. But that adds complexity, which means that more chips out of every batch will be defective, which means lower yield from each silicon wafer, which means more expensive sensors. Also, all that extra circuitry takes up space that could better be devoted to more or larger pixels. So what they do instead is to build in enough memory cells to image a single row all at once. That row is sent to the processor and then the next row is read, and so on. This is the rolling shutter.

Answer 2

Because the sensor essentially never stops sensing. There is no mechanism built into the sensor to not bleed charge from the little capacitors when light hits it. It also takes long enough to read out all the data from the sensor, so that there would be significant exposure time variation between parts of the image if the firmware reset the sensor, waited for the exposure time, then read out the data.

Digital sensors can read successive frames of video without needing mechanical shutters. However, the frame rate is fixed and known, the exposure time per frame is quite long (relative to typical still photography values), "rolling" exposure where different parts of the frame are exposed at different times is acceptable, and resolution is low.

Some sensors do essentially have "electronic shutters". These can integrate the light for a specified time, then freeze the value at least long enough for the whole array to be read out. However, this takes silicon area, which is usually more desirable to spend on better light gathering in digital still cameras.

To get the accurate and fast shutter times and the high resolution expected of today's digital cameras requires a mechanical shutter with current technology at prices that the market will accept.

Answer 3

The reason is economical. CMOS sensors with global shutter are available, but the added complexity makes them very expensive, and this extra cost would be justified for very, very few photographers.

Let's take Sony video camera models PMW-F5 vs. PMW-F55. Global shutter is the main difference between them. The price difference is quite steep, $16k vs $29k - and that's only for a sensor spitting out 9 megapixels. In addition, light sensitivity of the F55 suffers by almost a stop.

Global shutter comes as granted with frame-transfer CCD sensors. CCD's weak spot is high-ISO performance, so they are used only in some higher price tier cameras, such as some Leica models and medium format.