What's the Point of Shutterspeed?

Q. In the tech specs of your camera reviews, you always list the maximum shutter speed, which can be 1/1,000 or even faster. What is the point of changing the shutter speed, since the camera is going to be recording 60 fields (30 frames) per second anyway?

Frank Ducatti

Internet

A. As you probably know, the camcorder doesn’t actually have a shutter, just a Charge-Coupled Device (CCD) that takes a picture (dumps its data) every 1/60th of a second. As you note, the CCD takes only half a picture (i.e. a field) every 1/60th of a second. A camcorder combines two fields into a frame, at a rate of 30 frames per second.

The shutter speed control does not change the sample rate of the CCD, which remains constant at 60 samples a second. Instead, the shutter speed control sets the duration for which the chip is read. When the shutter speed is 1/120, for example, the camera still reads the CCD every 1/60th of a second, but only for the first half of that period. When the shutter speed is slower than 1/60, it allows the chip to remain partly charged, building intensities from one 1/60-second cycle to the next. The October 1998 issue of Videomaker has a good introductory article on the subject. Sure, all of this is interesting, but what you really want to know is: how does shutter speed affect my videography in the real world? First, shutter speed affects the exposure. As you increase the shutter speed, your picture gets dimmer. Unlike stopping down the iris, however, this method of dimming the picture does not increase the depth of field. Remember this when you want to blur the background behind a face: leave the iris open wide, but play with the shutter speed adjustment until you get a correct exposure.

Second, shutter speed affects motion blur. If you want to eliminate some of the blur you get when shooting fast-moving objects, or if you want to grab a sharp still from a frame of video, increase the shutter speed. Golfers sometimes analyze their swings by looking at video of these shot at fast shutter speeds. Conversely, slow the shutter speed to add blurriness when this is the effect you desire. Figure 1 shows frames shot of a fast-moving object at shutter speeds ranging from 1/1,000 at the top to 1/60 at the bottom. The object becomes more distinct as shutter speed increases, until we see it as a clearly defined object at two distinct locations when each of two fields were sampled.

Leaving shutter speed at the default 1/60 setting forces you to control exposure with the iris alone: like insisting on using only a hammer, when sometimes a ratchet is what is called for.

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