Why do digital cameras use shutters?

Question

Most digital cameras have live view but a lot also have a (mechanical) shutter (my Ricoh GR for example). If the sensor is sending information continuously for the live view why can’t this data be used for creating the picture? And how does the operation in live view and “picture taking” mode differ?

In: Engineering

Answer 1

It takes time for a career to read out the pixels line by the Most sensor can reset a pixel and it captures light until you read it. So your fist capture and read the top line and then capture the next line of pixels a bit later and so on. The result is what is called a Rolling Shutter Effect. Look at [this video for a good explanation and demonstration of it.](https://www.youtube.com/watch?&v=dNVtMmLlnoE)

So if you like to capture an image with minimal rolling shutter effect you can have a global electronic shutter the requires extra transistors and another part on the sensor that increases cost and decreases some other performance like light sensitivity. High-speed cameras use them but not regular consumer cameras.

Another option is to use a mechanical shutter. You reset the while sensor and use the mechanical shutter to control when the sensor is exposed to light. So you can expose all of it and then start to read out each line.
You still have a rolling shutter effect but it is reduced. It is a compromise to get better performance than just an electronic rolling shutter but a lower cost than a true global shutter.

If you look at your camera a Ricoh GR the max resolution is 4928 x 3264 and it captures video at 1920 x 1080. The result is that only a third of all line of pixels is needed and half of all pixels per line so you only read out 1/6 of the number of pixels. The result is that the rolling shutter effect is a lot less at the lower resolution.

It is also a video so you do not look at a single frame but the total effect of all frames so effect like the rolling shutter problem is less relevant.

So you could capture an image with an electronic rolling shutter as you do in video and lots of simpler cameras and I suspect all cellphones do it that way. The result is that the image in some condition would have more artifact then if you use a mechanical shutter.