SLR cameras have been a popular camera type for many decades
Single lens reflex, or SLR, cameras have been a popular type of camera for decades. Popular as 35mm cameras, medium format cameras using 120 film, and now digital cameras, SLRs solve a number of issues found in alternative camera forms.
What You See is What You Get
Unlike other camera types, with single lens reflex cameras, the image you see in the viewfinder is the image that is captured by the film or sensor. Light passing through the lens is reflected via a mirror to the camera's viewfinder. This image passes through a pentaprism or petamirror to correct the orientation of the image. Without this pentaprism or pentamirror, the image would appear upside down and reversed in the viewfinder. When the shutter is pressed, the mirror flips up allowing the light to pass through the shutter to the film or sensor.
Basic Controls
Cameras are simple. Although there are cameras available with automated controls, the most basic controls on a camera are simple. These basic controls have one purpose: To control the amount of light that reaches the film or digital sensor. The first control is the amount of time the shutter remains open. The longer the shutter remains open, the more light falls on the film or sensor. The second control is called the "aperture." The aperture is an opening in the lens that can be enlarged or reduced. The smaller the hole, the less light that falls on the film or sensor. It is a little counterintuitive, but the smaller the number of the aperture, the larger the opening. For example, f/2.8 allows more light to pass through the lens than f/22. A combination of these two controls, along with the shutter release and lens focusing ring, are all that is needed in an SLR camera.
Automated Controls
Modern automated SLR cameras offer a number of features that make it faster and easier to take pictures. Most automated SLRs allow the photographer to override all automation and operate the camera manually. However, most photographers prefer to set either the aperture or shutter speed and have the camera calculate the proper exposure. For example, by selecting a preset aperture, the camera's light meter will look at the overall exposure and select the correct shutter speed for a proper exposure. If "shutter-priority" is selected, the aperture is adjusted automatically while the shutter stays at a fixed setting. Some cameras have a "program" mode where both aperture and shutter speed are adjusted based on the light and type of lens attached to the camera.
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