Basics of SLR Cameras
Single-lens reflex cameras are one of the major categories of still cameras for amateur and professional photographers. They can be film-based or digital. Single-lens reflex cameras employ a single lens and an automatically moving mirror that allows the photographer to sight the image through the lens exactly as it will appear in the final photograph.
Basic Operation
The single-lens reflex camera takes its name from the use of a single lens that acts as both the viewer when sighting the image through the viewfinder and the lens that focuses the image on the film planes for exposure. Allowing the lens to serve both purposes is a reflex mirror. This small mirror is mounted at a 45-degree angle inside the camera body. When the mirror is in its "down" position, it blocks the light entering the lens and redirects it to the viewfinder. Thus, when the photographer looks into the viewfinder he is essentially looking out through the lens, seeing exactly what it is pointed at from the same angle.
When the shutter is engaged, the mirror flips up, allowing light from the lens to strike the film plane. A curtain shutter (sometimes a leaf shutter) draws open, exposing the film for the selected duration. When the exposure is complete, the shutter closes and the mirror returns to its down position.
The Pentaprism
One major technological element essential to single-lens reflex cameras is the pentaprism.This is a glass prism with five reflective faces. Light is directed from the reflex mirror upward into the pentaprism where it is redirected to the viewfinder by means of a series of reflections. This makes it possible for the user of a single-lens reflex camera to view his subject through a viewfinder located on the back of the camera, rather than being forced to look downward into the camera or from some other difficult angle.
Since the light as seen in the viewfinder has been reflected several times, the image seen by the photographer is generally darker and duller than the light actually entering the lens. An experienced photographer will learn to compensate for the loss of quality of light from the pentaprism.
SLR Advantages
The biggest advantage of a single-lens reflex camera is the fact that the photographer sees the image in the viewfinder precisely as it will be captured on film. Since he is sighting and lining up the subject through the camera's lens, he can frame the image to his liking without the need to correct for the small variations that occur between the image seen by the viewfinder and the image seen by the lens in a rangefinder camera, where the two elements are separate.
Additionally, the photographer sees his manipulation of the zoom and focus as they will appear in the finished photograph. Again, this advantage comes from looking directly through the lens. It allows photographers to manipulate the zoom and focus without needing to measure their distance to the subject or correct for filters or other accessories that might change the quality of light.
SLR Disadvantages
Ironically, the major disadvantage of a single-lens reflex camera also comes from the fact that the image is viewed through the lens. Because of the reflex mirror, the image seen in the viewfinder is not visible during the moment of exposure, since the mirror swings up to reveal the film plane, thus blocking the viewfinder. This means that the photographer sees blackness at the instant the image is exposed. It can be problematic in certain situations, such as photographing high-speed action.
In situations like this, a rangefinder camera may be more appropriate. Although the image seen in the viewfinder is from a slightly different angle than the one seen by the lens, there is no interruption of the image and the photographer can continue to observe his subject as the film is being exposed.
The complex optical system involved in an SLR camera also means that the cameras are typically more bulky than other camera styles and have more components that are capable of breaking down over time.
Variations
Single-lens reflex cameras are very popular and come in a variety of styles. They can feature either manual or auto-focus lens systems. Also, many SLR cameras use internal light metering sighted through the lens. This may be part of an automatic exposure setting or simply an aid for the photographer in choosing the proper exposure.
Although the majority of SLR cameras use 35mm film, SLR cameras have been produced in a variety of formats, including 110 film. Since the late 1990s, digital SLRs have been popular and are produced by most major camera companies.
Tags: image seen, through lens, reflex camera, reflex cameras, seen viewfinder