Have you ever used a film camera and noticed that photo you took is slightly off-center from the image you framed through the viewfinder, especially on close-up shots? This happens because one cannot look through the actual lens of the camera—the shutter and film (or in the case of digital cameras, the image sensor) are in the way. Thus, the viewfinder is off to the side of the actual camera lens, resulting in two slightly different angles of view.



SLR cameras fix this problem by using mirrors and prisms to let the photographer see through the camera lens. It achieves this by lowering a mirror into the lens box. The mirror reflects the image coming through the lens upward into a prism, which in turn bounces the image into the viewfinder. This way, the photographer is able to see exactly what the camera lens is seeing, with the exception of the brief instant when the photograph is being taken.

At this point, the mirror flips up out of the way of the lens box. The shutter opens, exposing the film or image sensor to the light coming through the lens. The picture is captured, the shutter closes, the mirror drops back down, and the photographer can once again see through the viewfinder.

The introduction of digital “point and shoot” cameras changed the game slightly. They also allow the photographer to see exactly what the lens is seeing. They have limitations, however, that some professional photographers feel make them unusable. The main drawback of commercial digital cameras is a considerable lag time between when the photographer pushes the button and when the camera actually captures the image. During these seconds, the camera may be jostled, the angle change, the lens shift out of focus, or the subject being photographed may move. Although perfectly fine for still pictures, photographers who need to capture an image instantly, such as sports, action, or wildlife photographers, the lag time is unacceptable.

A new generation of Digital SLR cameras (DSLR) combine the best of both technologies. They use the SLR mirror system to allow the photographer to see exactly what the lens sees. Simply by replacing the film behind the shutter with a digital light image sensor instead, a DSLR camera is still able to capture images instantly. In this way, a DSLR camera combines the accuracy and speed of an SLR camera with the convenience of digital cameras.

As DSLR cameras are usually high-end machines intended for a professional market, they tend to be more expensive, but also incorporate other advanced features. An average DSLR camera usually includes auto focus options, live preview, electronic flash controls, the ability to swap specialized lenses, and electronic adjustment of the captured digital image, such as optimizing the contrast and color, red-eye correction, and monochrome options, among many others. DSLR cameras also generally have a larger digital image sensor, providing better quality images with a higher pixel resolution, lower noise, and a superior color range. While the price of DSLR cameras make them a financial investment, professional photographers and amateur enthusiasts will consider it well worth it.