There are three primary types of video camera shutter: mechanical, electronic, & rolling.
In a film camera, there must be a mechanical shutter to close off the light path while the film is being
advanced to the next frame, the film being held still in the image plane for the duration of the exposure.
The shutter is a two-bladed fan, rotating at half the frame rate, such that first one blade closes the light
path, then the other. The width of the blades can usually be varied for effect, the open period being measured
as an angular rotation, with 360 meaning fully open, thus a normal 50% shutter is 180. Actually, this isn't
always true: a rotating prism can be used instead of a shutter, directing incoming light onto the correct frame
of the continuously moving film. But that's another story.
Video Camera Shutters
Video cameras generally do not need a mechanical shutter, with the exception of the Frame Transfer CCD. Some
Thomson cameras have a mechanical shutter for this reason. In television, the use of a shutter will, with this
exception, be an artistic tool. The shutter defines the exposure period; the normal, un-shuttered condition is
for the exposure period to be exactly the reciprocal of the field rate in an interlaced camera or frame rate in a
progressive camera.
Video cameras achieve shuttering by dumping the exposure. The exposure period normally starts at the
beginning of a field or frame, and ends at the end of the field or frame, when the charge is read out to form
the video signal. But if, exactly half way through this period, the charge is all read out and then ignored,
the charge accumulating for the remaining period has an exposure duration of 50% of the interval, 180, just as
in a film camera.
Digital Shutters
Before CCD and CMOS sensors, all video cameras used scanning tubes as the sensors. The scanning spot simply
discharged the light-sensitive target layer of the tube, line by line, pixel by pixel. This is effectively a
rolling shutter, and in the days when all displays were cathode ray tube based, the camera spot and the display
spot were always in synchronism, and all was well since the display effectively also had a rolling shutter.
However, as pixel-based sensors (CCD and then CMOS) replaced tubed cameras, an odd effect was noticed: on rapid
motion, moving edges sloped. This was due to the mismatch between the fixed shutter in the camera and the
rolling shutter in the display. The advent of pixel-based displays (plasma and LCD) has cured that and all is
well again, but some cameras are reintroducing the rolling shutter for convenience in the processing, and
sloping edges are once again on the menu. There is no cure for this, except for ensuring that both the camera
and display use the same kind of refreshing mechanism.
Contributed by Alan Roberts