Why Digital Cameras Have Mechanical Shutters
Ever wondered why digital cameras, particularly high-end digital SLR's, have mechanical shutters? The sensor is electronic, so why can't it be told to simply sample the light for the length of time specified by the shutter speed? Why can't the sensor just start accumulating light (what is sometimes referred to as a "charge"), wait a specified length of time, and then stop accumulating light at the end of the exposure time? Let's take a quick look at the reason mechanical shutters are used in digital cameras.
The Shutter Itself
Digital cameras use several different types of mechanical shutters, but all of them serve the same purpose. They block light from reaching the sensor when closed and move out of the way to let light accumulate on the sensor while open. Of course, the first thing that comes to mind is that the sensor, being an electronic device, should be able to simply turn on/off electronically. Why is the shutter even needed? Well, in fact, many cameras do use an electronic shutter that simply turns on/off the "light reading" capability of the sensor when needed. Many pocket point-and-shoot cameras use this technique. Pocket cameras that use the rear LCD to preview the picture are sometimes set up this way and hence have no mechanical shutter at all. Realizing that some cameras have all-electronic shutters while others have mechanical shutters, it's obvious that there are pros and cons to both designs.
Cameras, typically smaller point-and-shoot cameras, that use no mechanical shutters typically use an interline transfer sensor. An interline transfer sensor dedicates a portion of each pixel to store the charge for that pixel. The added electronics necessary to be able to store the charge for each pixel reduces the fill factor of the pixel, in turn reducing it's ability to capture light since a portion of each pixel is not light sensitive. Microlenses can be used to compensate but they are not 100% efficient and they can add expense to the design. Interline transfer sensor's typically have higher noise levels and lower sensitivity than the full frame sensor's used in high end digital SLR's. One obvious benefit is that this design eliminates the need for a potentially bulky mechanical shutter and can turn a purse size camera into a shirt pocket camera.
Digital cameras that use a mechanical shutter typically use a type of sensor called a full frame sensor. Unlike the interline transfer sensor (above), the full frame sensor has no circuitry on the pixel to store the charge that builds up as light contacts the array. Cameras that use a mechanical shutter typically bleed off any residual electrical charge while the shutter is closed, open the shutter, and then close the shutter. Once the mechanical shutter is closed, circuitry is then used to shift the charge from each pixel into a storage area. Since the pixels on the sensor remain "live" during readout, if the shutter remained open, light would continue to alter the charge accumulated by each pixel during the shifting operation which could result in blur or ghosting.
Mechanical shutters: the bottom line
In layman's terms, a mechanical shutter is used to control how long the pixels on an image sensor collect light. A simple mechanical shutter can be used to turn the entire sensor array on/off during the exposure. This eliminates the need for added electronics at each pixel location that would be used to turn on/off the pixel and store the charge (accumulated light). By using a mechanical shutter, a simpler, less expensive, and more efficient sensor can be used: one that has a higher fill factor (uses more of each pixel to actually capture light). Of course, nothing is ever cut and dried. Some cameras use both a mechanical and an electronic shutter! In these cases, the electronic shutter is used to supplement the mechanical shutter by providing features like a faster flash sync speed where mechanical shutters are just not fast/accurate enough. Most digital SLR cameras that use a mechanical shutter, however, use the mechanical shutter to control the amount of charge accumulated on the sensor as this simple mechanical device can be used to simplify the circuitry on the sensor itself thereby generally improving image quality and reducing noise.
This article is designed to answer the question of why a digital camera, admittedly a "solid state" device that shouldn't logically need any moving parts other than a focus mechanism would need a mechanical shutter. The answer, on the surface, turns out to be relatively simple and I hope I've answered the question so that most people can grasp the concept.
-- Mike Chaney