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Single-lens reflex camera


A single-lens reflex camera (SLR) is a camera that typically uses a mirror and prism system (hence "reflex" from the mirror's reflection) that permits the photographer to view through the lens and see exactly what will be captured. 
With twin lens reflex and rangefinder cameras, the viewed image could be significantly different from the final image. When the shutter button is pressed on a mechanical SLR, the mirror flips out of the light path, allowing light to pass through to the light receptor and the image to be captured.
Who created the SLR camera?
Thomas Sutton











The photographic single-lens reflex camera (SLR) was invented in 1861 by Thomas Sutton, a photography author and camera inventor who ran a photography related company together with Louis Désiré Blanquart-Evrard on Jersey. Only a few of his SLR's were made.
Louis Désiré Blanquart-Evrard






What is meant by SLR camera?


A single-lens reflex camera (SLR) is a camera that typically uses a mirror and prism system (hence "reflex" from the mirror's reflection) that permits the photographer to view through the lens and see exactly what will be captured.

Prior to the development of SLR, all cameras with viewfinders had two optical light paths: one path through the lens to the film, and another path positioned above (TLR or twin-lens reflex) or to the side (rangefinder). 
Because the viewfinder and the film lens cannot share the same optical path, the viewing lens is aimed to intersect with the film lens at a fixed point somewhere in front of the camera. 
This is not problematic for pictures taken at a middle or longer distance, but parallax causes framing errors in close-up shots. 
Moreover, focusing the lens of a fast reflex camera when it is opened to wider apertures (such as in low light or while using low-speed film) is not easy.

Most SLR cameras permit upright and laterally correct viewing through use of a roof pentaprism situated in the optical path between the reflex mirror and viewfinder. Light, which comes both horizontally and vertically inverted after passing through the lens, is reflected upwards by the reflex mirror, into the pentaprism where it is reflected several times to correct the inversions caused by the lens, and align the image with the viewfinder. When the shutter is released, the mirror moves out of the light path, and the light shines directly onto the film (or in the case of a DSLR, the CCD or CMOS imaging sensor).

pentaprism


A pentaprism is a five-sided reflecting prism used to deviate a beam of light by a constant 90°, even if the entry beam is not at 90° to the prism. The beam reflects inside the prism twice, allowing the transmission of an image through a right angle without inverting it (that is, without changing the image's handedness) as an ordinary right-angle prism or mirror would.
 Without the pentaprism, the single reflex camera might never have been such a success. It enables photographers to see through their lenses via an eye-level viewfinder with everything the right way up and the right way round.Cameras use what’s called a ‘roof’ pentaprism, because it looks like a roof from certain aspects, and while the cross-section of a pentaprism is bound by five sides, it actually has seven or eight.All reflex cameras use a mirror placed at 45° behind the camera lens, and this projects the lens view onto a ground-glass screen viewed perpendicularly to the optical axis of the lens. Before the pentaprism was introduced, the photographer typically had to look downwards to view what the lens was viewing forwards. Unlike a plate camera, the view is projected the right way up, but it’s laterally reversed – not ideal for moving subjects. This is known as a waist-level reflex viewfinder because it was often most convenient to place the camera at waist height when shooting.


Through-the-lens light metering


In photography, through-the-lens (TTL) metering refers to a feature of cameras whereby the intensity of light reflected from the scene is measured through the lens; as opposed to using a separate metering window or external hand-held light meter. In some cameras various TTL metering modes can be selected. This information can then be used to set the optimal film or image sensor exposure (average luminance), it can also be used to control the amount of light emitted by a flash unitconnected to the camera.
Through-the-lens light metering is also known as "behind-the-lens metering". In the SLR design scheme, there were various placements made for the metering cells, all of which used CdS (Cadmium sulfide) photocells. The cells were either located in the pentaprism housing, where they metered light transmitted through the focusing screen; underneath the reflex mirror glass itself, which was Topcon's design; or in front of the shutter mechanism, which was the design used by Canon with their Canon Pellix.

Semi-automatic exposure capabilities

Autoexposure, technically known as semi-automatic exposure, where the camera's metering system chooses either the shutter speed or the aperture, was finally introduced by the Savoyflex and popularized by Konishiroku in the 1965 KonicaAuto-Reflex. This camera was of the 'shutter-priority' type automation, which meant that the camera selected the correct aperture automatically. This model also had the interesting ability to photograph in 35 mm full-frames or half-frames, all selected by a lever.

Full-program auto-exposure
Full-program auto-exposure soon followed with the advent of the Canon A-1 in 1978. This SLR had a 'P' mode on the shutter speed dial, and a lock on the aperture ring to allow the lens to be put on 'Auto' mode. Other manufacturers soon followed with Nikon introducing the FA, Minolta introducing the X-700 in 1981,and Pentax introducing the Super Program. Olympus, however, continued with 'aperture-priority' automation in their OM system line.

The 1970s and 1980s saw steadily increasing use of electronics, automation, and miniaturization, including integrated motor driven film advance with the Konica FS-1 in 1979, and motor rewind functions.

Autofocus



An autofocus (or AF) optical system uses a sensor, a control system and a motor to focus on an automatically or manually selected point or area. An electronic rangefinder has a display instead of the motor; the adjustment of the optical system has to be done manually until indication. Autofocus methods are distinguished by their type as being either active, passive or hybrid variants.




Shutter Mechanisms 

Focal-plane shutters


Almost all contemporary SLRs use a focal-plane shutter located in front of the film plane, which prevents the light from reaching the film even if the lens is removed, except when the shutter is actually released during the exposure. There are various designs for focal plane shutters.







Leaf shutters

Another shutter system is the leaf shutter, whereby the shutter is constructed of diaphragm-like blades and can be situated either between the lens or behind the lens. If the shutter is part of a lens assembly some other mechanism is required to ensure that no light reaches the film between exposures.

An example of a behind-the-lens leaf shutter is found in the 35 mm SLRs produced by Kodak, with their Retina Reflex camera line; Topcon, with their Auto 100; and Kowa with their SE-R and SET-R reflexes.
A primary example of a medium-format SLR with a between-the-lens leaf shutter system would be Hasselblad, with their 500C, 500CM, 500 EL-M (a motorized Hasselblad) and other models (producing a 6 cm square negative). Hasselblads use an auxiliary shutter blind situated behind the lens mount and the mirror system to prevent the fogging of film.

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