The digital camera shutter release sets off a chain reaction.
In a digital camera, the film has been replaced by an electronic marvel called a charge-coupled device (CCD). The amount of light striking the CCD is controlled by an opening in the lens, called an aperture, and a shutter that allows light to strike the CCD. There are two kinds of shutters in digital cameras, and while they both serve the same purpose, they operate differently.
Sensor Exposure
In digital cameras, the exposed light is captured on the CCD. The surface of this sensor is covered with thousands of photosites, small areas that capture the brightness for a single pixel in the image as minute electrical charges. The sensor is called a coupled-device because it is coupled to a storage area. When you press the camera shutter release, the sensor is exposed to light; when the exposure is complete, the charges stored in each photosite are converted to binary data and processed.
Image Exposure
The amount of light reaching the CCD is called the exposure, which is controlled by two factors: the aperture and the shutter speed. The aperture is a variable-sized hole in front of the lens optics that adjusts to let more or less light through. The shutter can be of two types: an electronic shutter that turns the CCD on and off during the image exposure or a mechanical shutter.
Electronic Shutters
In an electronic shutter system, the sensor dedicates a portion of each sensor photosite to store the charge for that pixel. This reduces the pixel's ability to capture light, since a portion of the pixel is not light-sensitive, but is holding the electrical charge for that pixel. Electronic shutters often have lower sensitivity, but the design eliminates the need for a mechanical shutter--which means cameras can be very small.
Mechanical Shutters
In most film cameras, a shutter curtain opens at the film plane for the amount of time calculated for the exposure. This is called a focal plane shutter. A key advantage of this type of shutter is that it makes it easier to design a system with removable lenses. Digital cameras that use a mechanical shutter don't use the pixels on the CCD to store the charges that result from light striking the sensor, making more of each photosite available to capture light information from the scene. This generally results in improved image quality.
Controlling the Exposure
Using a film camera, a photographer chooses a lens aperture and a shutter speed. Each aperture opening is half as large as the previous aperture, designated as f/8, f/11, f16 and so on. The same is true for shutter speeds: each speed is twice as fast as the previous speed, measured as 1/125th of a second, 1/250th of a second and so on. With digital shutters, the exposure can be determined by the computer in the camera, and the shutter and aperture set accordingly. You might end up with an aperture of f/9.765 and a shutter speed of 1/387th of a second.
Tags: aperture shutter, aperture shutter speed, shutter speed, shutter that, amount light