A global shutter is something applicable to a video camera typically. It’s purpose is to overcome that jello effect that occurs when you pan your video camera left and right in a scene. This effect is referred to as ‘rolling shutter’.
To understand what it is we need to look back to film and then the digtial technologies that are now in use.
First, a little terminology…
You will often see, on the higher end cameras, and in snootier conversations with people trying to impress you with their knowledge, reference to a shutter angle. This is confusing, but it need not be. So, what is shutter angle? Shutter angle refers to the amount of time the shutter is open as it relates to the frame rate. If you are shooting at 24 frames per second, and you use a 1/24th shutter speed, that is a 360 degree shutter. Imagine the frame rate as a circle. The full circle is whatever the frame rate is. So if you are shooting at 24 fps the circle represents 1/24th of a second. The angle refers to the amount of that circle you will have the shutter open. If around 1/50th of a second (half the frame rate) this would be a 180 degree shutter. 1/100th would be a 90degree shutter, and so on. The shutter, physically, does not change angles within the camera, it is just a way to communicate camera configuration. It should be noted also that a 180 degree shutter allows half as much light to fall on the film/sensor as a 360 degree shutter, but that would be obvious once you associate that to a shutter speed.
In the film days you had a film based frame sitting behind a shutter. The shutter would open, the film would be exposed for however long it was going to be exposed, and the shutter would close. The film is then advanced to the next frame and exposed again, repeating this cycle 24 frames per second. During the time the shutter is closed no data is being captured by the film since the shutter is blocking the light from reaching the film.
In digital cameras though there is no physical shutter mechanism. Instead the data is pulled from the sensor. If you are running a 360 degree shutter (unusual) you would simply pull all the data accumulated since the last exposure, so every 1/24th of a second the exposure data is downloaded to memory. If running a 180 degree shutter (1/50th of a second) you would see the camera alternate between purging accumulated data and then collecting and downloading data.
Digital – CCD:
The early mainstream digital video cameras were based on the CCD technology. This, in many ways, replicated film with regard to how the data was captured. The entire face of the sensor is pulled into memory at once. This mimicks the way film would work. Thus, CCD based cameras do not suffer from the jello effect of rolling shutter.
Digital – CMOS:
Most digital cameras you will encounter are going to be CMOS sensor cameras. One key difference with a CMOS sensor is that the data is read from it in a scrolling fashion, from one edge to the other, usually bottom to top (remember the image is upside down, so the image is read top to bottom). This means that there is a temporal skew affecting the resulting image. Im there is something moving in the frame it will be in a different place in the image between the time the sensor read begins, and when it completes. So it will look crooked or squished, or stetched, depending on the kind of motion. You can see this easily. Take your smart phone camera and video something with vertical lines, like a building or pole. Pan the camera back and forth fairly rapidly. Play back the video and pause it. The pole will be leaning one way or the other.
What a global shutter does is mimic the shutter of the old film camera. At normal shutter rates, less than 360 degrees, it blocks the light from reaching the cmos sensor. This way the sensor reading process is not pulling in a changing data set. A global shutter is a really simple thing to implement but this feature is held out for the higher priced cameras and is normally a pricey add on. There are two ways this is implemented. Either an LCD panel that goes opaque to block the light, or a polarizer with a crystal behind it that changes polarity on command, which effectively blocks the light (2 polarizers in opposition will block 100% of the light). Some cameras, like the BlackMagic Ursa line, incorporate this at a low price point, while others hold it out for their highest product offering.
Ultimately, do you need a global shutter?
If you shoot carefully to avoid the effect, no, not really. This means no whip pans, no shooting out the side of a vehicle where you will see vertical poles or buildings filling the frame, that sort of thing. A small amount of rolling shutter can usually be corrected in post, but avoiding this is always a good idea. I would like to see global shutters find their way into the lower end cameras, perhaps this will happen soon, but I doubt it. Global shutter is a premium item still, so most people just learn to accomodate it. If they absolutely must have this feature then renting the equipment is the next best choice.