Single-lens reflex camera

As a small matter of history, the first 35 mm camera (non-SLR) to feature through the lens light metering may have been Nikon, with a prototype rangefinder camera, the SPX. According to the website below, the camera used Nikon 'S' type rangefinder lenses.[3]

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.

Pentax was the first manufacturer to show an early prototype 35 mm behind-the-lens metering SLR camera, which was named the Pentax Spotmatic. The camera was shown at the 1960 photokina show. However, the first Through-the-lens (TTL) light metering SLR on the market was the 1963 Topcon RE Super, which had the CdS metering cell placed behind the reflex mirror. The mirror had narrow slits cut into the surface to let the light reach the cell providing average metering. Late in the following year, a production model of the Pentax Spotmatic was shown whose CdS light meter cells were on the pentaprism, reading the light off the focusing screen providing average reading, yet keeping the Spotmatic name, but now written in one word. Another clever design appeared in 1965, the Canon Pellix employing a pellicle mirror that is semi-transparent, placing the meter cell on an arm swinging into the lightpass behind the mirror for meter reading.

Mamiya Sekor came out with cameras such as the Mamiya Sekor TL and various other versions. Yashica introduced the TL Super. Both of these cameras used M42 screw thread lenses as did the Pentax Spotmatic. Later on Fujica introduced their ST-701, then ST-801 and ST-901 cameras. The ST-701 was the first SLR to use a silicon cell photodiode, which was more sensitive than CdS and was immune to the memory effect that the CdS cell suffered from in bright sunlight. Gradually, other 35 mm SLR camera manufacturers changed their behind-the-lens meters from CdS cells to Silicon Diode photocells.

Other manufacturers responded and introduced their own behind-the-lens metering cameras. Nikon and Miranda, at first, simply upgraded their interchangeable pentaprisms to include behind-the-lens metering (for Nikon F, and Miranda D, F, Fv and G models) and these manufacturers also bought out other camera models with built-in behind-the-lens metering capability, such as the Nikkormat FT and the Miranda Sensorex (which used an external coupling diaphragm). Minolta introduced the SRT-101, which used Minolta's proprietary system they referred to as "CLC", which was an acronym for "contrast light compensation", which metered differently from an average metering behind-the-lens camera.

Some German manufacturers also introduced cameras such as the Zeiss Ikon Contarex family, which was one of very few 35 mm SLR to use interchangeable film backs.

Inexpensive leaf-shutter cameras also benefited from behind-the-lens metering as, Topcon introduced the Auto 100 with front-mount interchangeable lenses designed only for that camera, and one of the Zeiss Ikon Contaflex leaf shutter cameras. Kowa manufactured their SET-R, which had similar specifications.

Within months, manufacturers decided to bring out models that provided limited area metering, such as Nikon's Photomic Tn finder, which concentrated 60% of the CdS cells sensitivity on the inner circle of the focusing screen and 30% on the surrounding area. Canon used spot metering in the unusual Canon Pellix camera, which also had a stationary mirror system that allowed approximately 70% of the light to travel to the film plane and 30% to the photographer's eye. This system, unfortunately, degraded the native resolution of the attached lens and provided less illumination to the eyepiece. It did have the advantage of having less vibration than other SLR cameras but this was not sufficient to attract professionals to the camera in numbers.

While auto-exposure was commonly used in the early 1960s with various 35 mm fixed lens rangefinder cameras such as the Konica Auto 'S', and other cameras such as the Polaroid Land cameras whose early models used selenium cell meters, auto-exposure for interchangeable lens SLRs was a feature that was largely absent, except for a few early leaf-shutter SLRs such as the Kowa SE-R and Topcon Auto 100.

The types of automation found in some of these cameras consisted of the simple programmed shutter, whereby the camera's metering system would select a mechanically set series of apertures with shutter speeds, one setting of which would be sufficient for the correct exposure. In the case of the above-mentioned Kowa and Topcon, automation was semi-automatic, where the camera's CDs meter would select the correct aperture only.

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 Konica Auto-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.

Other SLRs soon followed, but because of limitations with their lens mounts, the manufacturers of these cameras had to choose 'aperture-priority' automation, where the camera's metering system selects the correct shutter speed. As one example, Pentax introduced the Electro Spotmatic, which was able to use the then considerable bulk of 42 mm screw-mount lenses produced by various manufacturers. Yashica, another screw-mount camera manufacturer, soon followed.

Canon, which produced the FD lens mount (known as the breech-mount; a unique lens mounting system that combines the advantages of screw-mount and bayonet-mount) introduced their shutter priority 35 mm SLR, the Canon EF in 1976 or so. This camera's build quality was almost the equal of their flagship camera, the Canon F1, and featured a copal-square vertically travelling focal plane shutter that could synchronize electronic flash at shutter speeds up to and including 1/125 of a second, thus making this a good second-body camera for the professional photographer.

Nikon at first produced an aperture-priority camera, but later made subtle changes on the inside of their bayonet mount, which allowed for shutter-priority automation without obsoleting the photographers lenses.

Minolta XD-11 (also XD-7 and XD)

Overview
Type	35 mm SLR
Lens
Lens mount	Minolta SR-mount
Focusing
Focus	Manual focus SLR
Exposure/metering
Exposure	Shutter and aperture priority autoexposure
Flash
Flash	Hot shoe only; no PC connector
General
Dimensions	51 × 86 × 136 mm, 560 g

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,[4] 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,[5] and motor rewind functions.

The first autofocus 35 mm SLR was the Pentax ME-F released in 1981.[6]

The Minolta Maxxum 7000, released in 1985, was the first 35 mm SLR with integrated autofocus and motorized film-advance winder, which became the standard configuration for SLR cameras from then on. This development had significant impact on the photographic industry.

Some manufacturers discarded their existing lens systems to compete with other manufacturer's autofocus capability in their new cameras. This was the case for Canon, with its new EOS lens line. Other manufacturers chose to adapt their existing lens systems for autofocus capability, as was the case with Nikon and Pentax. This allowed photographers to continue using their existing lenses, which greatly reduced the cost of upgrading. For example, almost all Nikon lenses from the 1960s and later still function on the current Nikon bodies, only lacking autofocus. Still some manufacturers, notably Leica with its R-system lenses, and Contax with its Zeiss lenses, decided to keep their lens mounts non-autofocus.

Typical film SLR viewfinder information

From the late 1980s competition and technical innovations made 35 mm camera systems more versatile and sophisticated by adding more advanced light metering capabilities such as spot-metering; limited area metering such as used by Canon with the F1 series; matrix metering as used by Nikon, exposure communication with dedicated electronic flash units. The user interface also changed on many cameras, replacing meter needle displays that were galvanometer-based and thereby fragile, with light-emitting diodes (LEDs) and then with more comprehensive liquid crystal displays (LCDs) both in the SLR viewfinder and externally on the cameras' top plate using an LCD screen. Wheels and buttons replaced the shutter dial on the camera and the aperture ring on the lens on many models, although some photographers still prefer shutter dials and aperture rings. Some manufacturers introduced image stabilization on certain lenses to combat camera shake and to allow longer hand-held exposures without using a tripod. This feature is especially useful with long telephoto lenses.

Canon, Nikon and Pentax have all developed digital SLR cameras (DSLRs) using the same lens mounts as on their respective film SLR cameras.[7] Konica Minolta did the same, and after having bought Konica Minolta's camera division in 2006 Sony continue using the Minolta AF lens mount in their DSLRs, including cameras built around a semi-transparent fixed mirror. Samsung builds DSLRs based on the Pentax lens mount. Olympus, on the other hand, chose to create a new digital-only Four Thirds System SLR standard, adopted later by Panasonic and Leica.

Contax came out with a DSLR model, the Contax N-Digital. This model was too late and too expensive to be competitive with other camera manufacturers. The Contax N-digital was the last Contax to use that maker's lens system, and the camera, while having impressive features such as a full-frame sensor, was expensive and lacked sufficient write-speed to the memory card for it to be seriously considered by some professional photographers.

The digital single-lens reflex camera have largely replaced film SLRs design in convenience, sales and popularity at the start of 21st century.

A perspective drawing showing how a roof pentaprism corrects a laterally reversed SLR image.

Most 35 mm SLRs use a roof pentaprism or penta-mirror to direct the light to the eyepiece, first used on the 1948 Duflex[8] constructed by Jenő Dulovits and patented August 1943 (Hungary). With this camera also appeared the first Instant-return mirror. The first Japanese pentaprism SLR was the 1955 Miranda T, followed by the Asahi Pentax, Minolta SR-2, Zunow, Nikon F and the Yashica Pentamatic. Some SLRs offered removable pentaprisms with optional viewfinder capabilities, such as the waist-level finder, the interchangeable sports finders used on the Canon F1 and F1n; the Nikon F, F2, F3, F4 and F5; and the Pentax LX.

Another prism design was the porro prism system used in the Olympus Pen F, the Pen FT, the Pen FV half-frame 35 mm SLR cameras. This was later used on the Olympus EVOLT E-3x0 series, the Leica Digilux 3 and the Panasonic DMC-L1.

A right-angle finder is available that slips onto the eyepiece of most SLRs and D-SLRs and allows viewing through a waist-level viewfinder. There is also a finder that provides EVF remote capability.

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. Early focal-plane shutters designed from the 1930s onwards usually consisted of two curtains that travelled horizontally across the film gate: an opening shutter curtain followed by a closing shutter curtain. During fast shutter speeds, the focal-plane shutter would form a 'slit' whereby the second shutter curtain was closely following the first opening shutter curtain to produce a narrow, vertical opening, with the shutter slit moving horizontally. The slit would get narrower as shutter speeds were increased. Initially these shutters were made from a cloth material (which was in later years often rubberised), but some manufacturers used other materials instead. Nippon Kōgaku (now Nikon Corporation), for example, used titanium foil shutters for several of their flagship SLR cameras, including the Nikon F, F2, and F3.

Other focal-plane shutter designs, such as the Copal Square, travelled vertically — the shorter travelling distance of 24 millimetres (as opposed to 36 mm horizontally) meant that minimum exposure and flash synchronisation times could be reduced. These shutters are usually manufactured from metal, and use the same moving-slit principle as horizontally travelling shutters. They differ, though, in usually being formed of several slats or blades, rather than single curtains as with horizontal designs, as there is rarely enough room above and below the frame for a one-piece shutter. Vertical shutters became very common in the 1980s (though Konica, Mamiya, and Copal first pioneered their use in the 1950s and 1960s, and are almost exclusively used for new cameras. Nikon used Copal-made vertical plane shutters in their Nikomat/Nikkormat -range, enabling x-sync speeds from 1⁄30 to 1⁄125 while the only choice for focal plane shutters at that time was 1⁄60. Later, Nikon again pioneered the use of titanium for vertical shutters, using a special honeycomb pattern on the blades to reduce their weight and achieve world-record speeds in 1982 of 1⁄4000 second for non-sync shooting, and 1⁄250 with x-sync. Nowadays most such shutters are manufactured from cheaper aluminium (though some high-end cameras use materials such as carbon-fibre and Kevlar).

One unusual design, the Olympus Pen half-frame 35 mm SLR system, manufactured by Olympus in Japan, used a rotary focal-plane shutter mechanism that was extremely simple and elegant in design. This shutter used titanium foil but consisted of one piece of metal with a fixed opening, which allowed electronic flash synchronisation up to and including its maximum speed of 1/500 of a second – rivalling the capabilities of leaf-shutter systems

Another 35 mm camera system that used a rotary shutter, was the Robot Royal cameras, most of which were rangefinder 35 mm cameras. Some of these cameras were full-frame; some were half-frame, and at least one Robot camera produced an unusual square-sized image on the 35 mm frame.

The Mercury II, produced in 1946, also used a rotary shutter. This was a half-frame 35 mm camera.

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, 500 cm, 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.

Other medium-format SLRs also using leaf shutters include the now discontinued Zenza-Bronica camera system lines such as the Bronica ETRs, the ETRs'i (both producing a 6 × 4.5 cm. image), the SQ and the SQ-AI (producing a 6 × 6 cm image like the Hasselblad), and the Zenza-Bronica G system (6 × 7 cm). Certain Mamiya medium-format SLRs, discontinued camera systems such as the Kowa 6 and a few other camera models also used between-the-lens leaf shutters in their lens systems.

Thus, any time a photographer purchased one of these lenses, that lens included a leaf shutter in its lens mount.

Because leaf shutters synchronized electronic flash at all shutter speeds especially at fast shutter speeds of 1⁄500 of a second or faster, cameras using leaf shutters were more desirable to studio photographers who used sophisticated studio electronic flash systems.

Some manufacturers of medium-format 120 film SLR cameras also made leaf-shutter lenses for their focal-plane-shutter models. Rollei made at least two such lenses for their Rolleiflex SL-66 medium format which was a focal-plane shutter SLR. Rollei later switched to a camera system of leaf-shutter design (e.g., the 6006 and 6008 reflexes) and their current medium-format SLRs are now all of the between-the-lens shutter design.

SLRs vary widely in their construction and typically have bodies made of plastic or magnesium. Most manufacturers do not cite durability specifications, but some report shutter life expectancies for professional models. For instance, the Canon EOS 1Ds MkII is rated for 200,000 shutter cycles and the Nikon D3 is rated for 300,000 with its exotic carbon fiber/kevlar shutter. Because many SLRs have interchangeable lenses, there is a tendency for dust, sand and dirt to get into the main body of the camera through the mirror box when the lens is removed, thus dirtying or even jamming the mirror movement mechanism or the shutter curtain mechanism itself. In addition, these particles can also jam or otherwise hinder the focusing feature of a lens if they enter into the focusing helicoid. The problem of sensor cleaning has been somewhat reduced in DSLRs as some cameras have a built-in sensor cleaning unit.

The price of SLRs in general also tends to be somewhat higher than that of other types of cameras, owing to their internal complexity. This is compounded by the expense of additional components, such as flashes or lenses. The initial investment in equipment can be prohibitive enough to keep some casual photographers away from SLRs, although the market for used SLRs has become larger particularly as photographers migrate to digital systems.