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All this wonderful process of collecting photons and converting them into digital information requires a specific time span for this to happen, known in the photographic realm as exposure time. Film cameras have always sliced light into manageable slivers of time using mechanical devices called shutters, which block the film until you’re ready to take a picture, and then open to admit light for the period required for (we hope) an optimal exposure. This period is generally very brief, and is measured, for most pictures taken with a hand-held camera, in tiny fractions of a second.

Digital cameras have shutters, too. They can have either a mechanical shutter, which opens and closes to expose the sensor, or an electronic shutter, which simulates the same process. Many digital camera have both types of shutter, relying on a mechanical shutter for relatively longer exposures (usually 1/500th second to more than a second long), plus an electronic shutter for higher shutter speeds that are difficult to attain with mechanical shutters alone. (That’s why you’ll find digital cameras with shutter speeds as high as 1/16,000th second: they’re electronic.)

Mechanical shutters can work with any kind of sensor. One important thing to remember about a digital SLR’s mechanical shutter is that its briefest speed usually (but not always) determines the highest speed at which an electronic flash can synchronize. That is, if your dSLR synchs with electronic flash at no more than 1/125th second, that’s probably the highest mechanical shutter speed available. Some special flash systems can synchronize with electronic shutters at higher speeds.

The type of electronic shutter your camera has depends a great deal on the kind of sensor that is built into your camera. In terms of the kind of shutter they can use, sensors fall into one of two categories: interline and full frame. Both terms deal with how the sensor captures an image.

The interline sensor, developed originally for video cameras, isolates an entire image in one instant, and then gradually shifts it off the chip into the camera electronics for processing and conversion from an analog signal to digital format. While this process is underway, a new image can be accumulating on the chip. That’s because the interline sensor is, in effect, two sensors in one; while one sensor is exposed to light, the other is masked. The two sensors exchange places so that the previously masked sensor can then accept light while the sensor that was previously exposed is shielded so it can offload its image to the camera’s electronics. This capability waimportant for video cameras, which expose their sensors at a rate of 30 frames per second.

Because of this ability to isolate an image in a fraction of a second, interline sensors can function as an electronic, non-mechanical shutter. A full-frame sensor (not to be confused with full-frame sensor size), in contrast, is a single sensor that cannot isolate an image while it is still exposed to light. The sensor must be physically covered, uncovered to make the exposure, and then covered again while the image is transferred to the camera’s electronics. If the sensor is still exposed to light when an image is moved from the chip, the image will be smeared by illumination that strikes the photosites while the old image is being shifted. That calls for a mechanical shutter.