Definition of SLR Camera

By Gwen Wark

A single lens reflex camera, commonly known as an SLR, is the type of camera used by professional photographers and serious hobbyists. The SLR uses a system of mirrors to bounce light from the lens to the viewfinder, ensuring that the image seen by the photographer is the image being captured on the film or digital image sensor. Single lens reflex cameras come in several varieties and configurations, including both film-based and digital camera format.


The SLR's history is reputed to have begun in 1884 with the invention of C.R. Smith's monocular duplex. The monocular duplex is the first camera known to have a mirror system which is mechanically moved during exposure, much like a modern SLR. This camera was a large-format camera, meaning the medium used for image capture was large and had to be inserted piece by piece as each exposure was taken. In the 1920s and 1930s, SLRs for smaller and more amateur formats were released, bringing the SLR to the general public.

How It Works

An SLR camera's work begins when the light enters the lens, before exposure. This light is reflected off of a mirror or prism system into the viewfinder of the camera. The setup allows the photographer to see the exact scene and lends a high degree of accuracy to imaging. When the shutter is depressed, the mirror is moved out of the way to allow the film or digital image sensor to be exposed to the light entering the lens. After the shutter curtain has closed, the mirror is returned to its initial position, ready for another exposure.

Types of SLR

The SLR camera is most commonly seen as used for a 35 millimeter format. This format of film is used by both amateurs and professionals, is readily available, and is easily processed. Medium format SLRs may use 120 format film or a larger image capture device. They come in many varieties, from the traditional hand held Hasselblad seen in the professional modeling and advertising fields to Mamiya's medium format digital SLR. Single lens reflex cameras may be either digital or film format.

The Digital SLR

The digital SLR is a reasonably recent development in the world of SLR cameras. Released by Kodak in 1991, the DCS-100 was the first commercially available digital SLR camera. The impact which this advancement would have on the world of professional and amateur photography alike was astonishing. It took imaging from an archaic chemical process to the digital darkroom, which in turn made SLR photography more accessible to amateurs. A digital SLR works in the same fashion as a standard SLR; the exception is that the film is replaced by a charge couple device (CCD) or complementary metal oxide semiconductor (CMOS) sensor located at the back of the camera.


Using an SLR gives a photographer a much greater control of the final image. With the ability to change lenses, single lens reflex cameras allow precise control of the shutter speed, depth of field, and width of the image. This allows images to be created in conditions that would defeat a point and shoot or fixed lens camera. Digital SLRs offer all of the benefits of film SLRs with the addition of the ability to shoot more consecutive images without stopping to change film.

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