Single-lens reflex camera

The single-lens reflex camera, more commonly known by the abbreviation SLR, uses a mirror placed between the lens and the film to project the image seen through the lens to a matte focusing screen. Most SLRs use a pentaprism to observe the image via an eyepiece, but there are also other finder arrangements, such as the waist-level finder.

This cross-section (side-view) of the optical components of an SLR shows how the light passes through the lens (1), is reflected by the mirror (2) and is projected on the matte focusing screen (5). Via a condensing lens (6) and internal reflections in the pentaprism (7) the image appears in the eye piece (8). When an image is taken, the mirror moves in the direction of the arrow, the focal plane shutter (3) opens, and the image is projected in the film (4) in exactly the same manner as on the focusing screen.

The shutter is almost always in the focal plane. If not, some mechanism is required to ensure that no light reaches the film between exposures.

Since the technology became widespread in the 1970s, SLRs have become the main type of camera used by dedicated amateur photographers, and professionals.

The major advantage of single lens reflex cameras is the ability to determine what parts of the image are in focus by looking through the taking lens itself. Many modern SLRs offer additional focussing aids, including rangefinder devices and automatic focussing methods. To save weight, many modern SLRs use mirrors instead of a pentaprism.

The SLR cameras also offer the ability for the photographer to quickly and simply change the lens, thus making them versatile. The same camera can be used for portrait, landscape, action, and close-up photography. Whilst some earlier non-SLR cameras offered this ability, the SLR immediately allows the photographer to see what the different lenses will capture.

SLR cameras also avoid the difficulty of parallax in close-up photography. Cameras with a separate optical viewfinder system show the scene from a different viewpoint from that of the camera lens. For photography at normal distances, this difference is not significant, but where the gap between the viewfinder and lens is a significant fraction of the distance to the objects in the scene to be photgraphed, this difference in viewpoint may mean that the image recorded is quite different from that seen through the viewfinder.

Typical 35mm SLR camera of the 1970s

The majority of SLR cameras use 35mm film, as this format offers a good compromise between image quality, size, and cost for most amateur and some professional applications. Professional photographers often use medium format SLRs for work where high image quality is important. Digital SLRs have also appeared on the market and are now the camera of choice for most newspaper photographers, but remain unaffordable for most amateurs.

The pros of SLR camera are:

Absence of parallax
Exact focusing, esp. important for macro and telephoto photography
Ability to check the depth of field (a feature often omitted on low-end SLR)
Vast range of interchangeable lenses (this feature is generally available also on high end rangefinder cameras, but the SLR concept is perfect for it)

The cons of SLR camera are:

Due to mechanical complexity, the SLR camera is larger than a rangefinder camera for a given film size.
Latency; the mirror needs time to move before the film is exposed. On modern SLR latency is generally increased by autofocus, and can be a real pain.
Noise and vibrations, mainly due to the mirror movement.
Dark viewfinder when using high f-number lenses.

History

Graflex produced SLR cameras as early as 1909. The modern 35mm SLR concept was introduced by the Exakta in 1936. Significant SLRs that set a standard were the Hasselblad and the Nikon F.