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A digital single-lens reflex camera (Digital SLR or DSLR) is a digital camera that operates on the same optical and mechanical principles as a modern electronic autofocus 35mm film single-lens reflex camera. The key difference is that the film is replaced with a CCD or CMOS sensor plus accompanying electronics, thus creating images digitally in-camera, without the need to first chemically develop the image captured.

All digital cameras produce images which are available instantly without the need for processing, can be transmitted and viewed electronically, and can be manipulated with image processing software. Film can be printed from a negative or projected optically, but must be scanned digitally for electronic transmission or storage.

A major attraction of digital SLR cameras as compared to non-SLR digitals has been to allow photographers to convert from 35mm film cameras using the lenses and equipment that they are familiar with. For this reason, camera manufacturers design digital SLRs to be as similar as possible to their film counterparts. This advantage is not inherent to the digital SLR, and in fact there is at least one non-SLR with the same features, the Epson Rangefinder Digital Camera R-D1, advertised as the world's first rangefinder digital camera.

Unfortunately, the delicate and complex mechanism of present DSLR cameras has a disadvantage as it makes them vulnerable to damage caused by shock, moisture, dirt, or sand, usually not covered by the camera's warranty. It remains questionable whether today's digital cameras can ever achieve, in mass volume, the 25-year lifespan of some older 35mm film SLR and rangefinder cameras of traditional design, such as the Nikon F3 or Leica.

The Pros and Cons of the Digital SLR

DSLR advantages compared to 35mm SLR film cameras:

• No costs for film purchase and processing
• Smaller and lighter image storage media
• Able to store hundreds of photos on each storage card
• Ability to change ISO speed
• No need for digital scanning of film media
• Immediate availability of photographs for transmission, viewing, etc.
• Ability to view and delete photographs as soon as they are taken

DSLR disadvantages compared to 35mm SLR film cameras:

• High initial cost
• Rapid obsolescence
• High total cost of ownership, compounded by their shorter useful lifetimes
• Dependence upon batteries and/or outside power sources for normal operation
• Dust collecting on the exposed sensor can degrade image quality
• Crop factor of more affordable DSLRs increases effective focal length and reduces ability to manipulate depth-of-field
• Film cameras can use different types of film for different purposes
• Complexity and shorter service life in extreme environments

SLR Design

The single-lens reflex (SLR) camera uses a mirror to show the image that will be captured on film in a viewfinder. At the time of taking the photograph, the mirror is moved out of the light path.

The principal advantage of the SLR film camera over non-SLR film cameras is the ability to see the image exactly as it would appear on film, whatever the lens used, and without parallax errors due to a viewfinder offset from the lens. While non-SLR cameras can have interchangeable optics, a viewfinder must be provided for each one. Close-up and macro photography are particularly affected by parallax.

Most of these advantages transfer directly to any digital camera with electronic viewfinder screen, even if not of SLR construction: the viewfinder always shows the image on the sensor, exactly as it will be captured in the photograph. Actually, both film SLRs and digital screens tend to show slightly more of the edges of the image than will be included. SLR viewing has the advantages over non-SLR digital screens of zero lag time and absence of heavy power drain, and is usually more robust and reliable.

In the digital era zoom lens design is sufficiently advanced to almost eliminate the market for non-SLR cameras with interchangeable lenses, so lens interchangability is, in practice, restricted to the digital SLR. A non-SLR camera can have a fixed lens which will zoom from medium wide-angle to medium telephoto, omitting only fish-eye and extreme telephoto. However, at least one advanced digital non-SLR camera with interchangeable lenses and large sensor has been produced.

The imaging sensor in a DSLR is typically much larger than the one in a consumer-level, compact digital camera. A larger sensor allows better image quality, lower noise, shallower depth of field and wider field of view. Most consumer-level DSLR sensors are APS-size, i.e., approximately 22 mm x 15 mm, the size of an APS film frame, smaller than a frame of 135 film. The angle of view of a lens depends upon its focal length and the image size; a sensor smaller than 35mm film means that a lens of given focal length will have a narrower angle of view than it would on 35mm film. If the sensor is the same size as the equivalent frame of film (36 mm x 24 mm), the camera is said to have a 35mm, or full-frame sensor. As of 2005 only a few expensive professional DSLRs have full-frame sensors. The Canon EOS-1Ds Mark II and the Canon EOS 5D are full-frame DSLRs. Medium-format size sensors, as used on the Mamiya ZD and other cameras, are even larger than 35mm or full-frame sensors, and capable of even greater image quality.

The impact of sensor size on focal length is referred to as the "crop factor", which is a multiplier that gives the full-frame-eqivalent focal length for a lens. APS-size sensors have a crop factor of 1.6, so a lens with a focal length of 100mm will give a FOV equal to that of a 160mm lens on a full-frame camera. This makes achieving long telephoto images on an APS-sensor camera much easier than on a full-frame camera; unfortunately, wide-angle views suffer by the same amount. Shallow depth-of-field images also tend to be more difficult to achieve with smaller sensors.

Depending on the position of the reflex mirror, the light from the scene can only reach either the eyepiece or the sensor. Also, a mechanical diaphragm is used to control the exposure. Therefore, even though a DSLR has a color display at the back of the camera body, it doesn't provide "live preview" (allowing focusing, framing, and depth-of-field preview using the display). The only exception (in 2005) is the Canon EOS 20Da, a Japanese variant of Canon EOS 20D designed for astrophotography.

Camera manufacturers currently expect that professionals and advanced amateurs will continue to prefer DSLRs over other forms of digital cameras. However, if electronic viewfinders with near-zero lag time, increased reliability, and very low power consumption are introduced, non-SLR cameras may become a viable choice for a large number of people who now need SLRs.

Many medium format roll-film SLRs can accept a digital camera back to turn the camera into a DSLR with very high image resolution and quality (typically 22 megapixel as of 2005). However, the combination is very expensive and bulky, and more suited to the studio than an outdoor environment.

History

In 1981, Sony unveiled a prototype of the company's first still video camera, the Mavica (Magnetic Video Camera). It recorded images on two-inch floppy disks and played them back via TV set or video monitor. The Mavica was not a true digital camera, but a form of TV camera equipped with a single-lens reflex (SLR) viewfinder with interchangeable lenses, capable of writing still photos onto magnetic disks. Sensor size was a meagre 570 x 490 pixels, on a 10mm x 12mm chip. The original Mavica had only one shutter speed, 1/60th second, and each image was recorded onto a Sony-design floppy disk, for a total of 50 photos. Images were displayed on a television set and were considered to be equal in resolution to the display capabilities of television sets of the era.

At Fotokina in 1986, Nikon revealed a prototype analog electronic still SLR camera, the Nikon SVC, a precursor to the digital SLR (Pierre J., "Nikon SLR-type digital Cameras", ). The prototype design lead to the Nikon N8008 two years later which is capable of 0.3 megapixel (ibid ).

In 1991, Kodak released the first comercially available digital SLR, the Kodak DCS-100. It consisted of a modified Nikon F3 SLR body, modified drive unit, and an external storage unit connected via cable. The camera was capable of producing 1.3 megapixel (1280x1024) and cost approximately $30,000 US. This was followed by the DCS-200 with integrated storage (Leefoo, "A Brief Info on Kodak DCS-Series Digital Still SLR cameras", ).

Over the next decade, digital SLRs have been released by various companies such as Canon, Nikon and Kodak with higher resolution (megapixel) and lower prices. Kodak has since ceased all DSLR production.

In 2003, Canon introduced a 6.3 megapixel Digital Rebel SLR camera (known in the UK as the EOS 300D) at an MSRP of under $1000. Its popularity, especially among newspaper and amateur photographers, encouraged other manufacturers to produce affordable digital SLR cameras, significantly lowering entry costs and allowing more casual photographers an opportunity to experience the digital SLR photography. Canon introduced the next generation 8 megapixel EOS350D (Digital Rebel XT) in 2005.

Kodak made a major impact on DSLR history by releasing not one, but two 14MP DSLR cameras for Nikon and Canon lens mounts respectively. These cameras were extremely expensive, and Kodak has since discontinued them. Konica Minolta briefly produced two DSLR models, the 5D and 7D, but has also ceased all DSLR production. At one time,Kyocera also manufactured DSLRs and marketed them under the Contax name, but in 2005 also withdrew from the digital-SLR camera field.

Present day

Canon and Nikon currently have the largest range of amateur and professional DSLR cameras. Canon's line includes the popular entry-level 350D, the advanced 5D (with a 35mm-size sensor), and the expensive, top-level 1Ds Mark II. Nikon has a broad line of DSLRs which, in 2005, includes the pro-level D2X, D2H as well as the amateur-level D200, D70s, and D50. Nikon has also concentrated on improving the weather resistance and sealing of its DSLR cameras, a necessary step towards improving DSLR reliability in extreme conditions. Fujifilm sells a Nikon-lens compatible DSLR, and Olympus has introduced two DSLR models of its own design. Sigma produces an innovative DSLR with the multi-layered Foveon X3 sensor to deliver excellent color and detail for its size and imaging limits, while Pentax also has digital SLRs that use their lenses and accessories. Mamiya has produced a medium-format SLR, the Mamiya ZD that currently produces the highest quality digital images, its larger sensor able to capture much more detail than the 35mm full-frame and smaller sensors found on the Canon, Nikon, Olympus, Pentax, and Sigma models.

Digital SLR versus Digital Compact and Prosumer Cameras

Non-SLR digital cameras are of two types, digital compacts, and SLR-like "prosumer" cameras with permanently fixed lenses, usually with zoom capability, e.g. Nikon 5700, Sony F series, Sony R1, Panasonic FZ series, Canon S2 IS). These lenses are usually faster in terms of lens speed than the zoom lenses fitted to less expensive compact digicams, sometimes capable of operation at f/2.8 over their entire zoom range.

Non-SLR digital cameras with long telephoto lenses (like Nikon 5700) also offer TTL viewing through the focusing lens, projected onto its viewfinder as well as the LCD screen, through an EVF(electronic viewfinder). This differs from the DSLR in that this is a digitally created image whereas a DSLR gives the real optical TTL image. An EVF image is slower to readjust (lag) to a change and of lower resolution than a true optical TTL, and because of the extra electronic complexity, prone to more electronic malfunctions. Advantages of EVF are parallax-free views compared with using an optical viewfinder separate from the lens, and far lesser bulk and mechanical complexity than a DSLR with its reflex viewing system. The disadvantage of a prosumer digital camera, of course, is its inability to change lenses because of its fixed lens mount. This also creates an issue when dust enters the camera and must be cleaned from the sensor. (On the other hand, while DSLR sensors are easier to clean, they're more likely to need cleaning since the removable lens system admits more dust).

Compact digital cameras can usually be operated at arm's length using only the LCD display screen at the rear of the camera, and most models also have simple optical viewfinders like traditional compact film cameras with viewfinders. Like the prosumer digital cameras, nearly all compact digicams have no ability to accept interchangeable lenses, with the exception of the (Epson R-D1 Rangefinder ). Most digicams are therefore provided with a zoom lens intended to cover most normally used focal lengths. Digicams may be altered through the use of supplementary add-on lens converters to provide an added telephoto or wide angle field of view, though the image quality is usually affected to a significant degree. Most compact digicams are significantly slower in shutter speed capture (time from image capture to sensor and storage) than DSLR cameras, a disadvantage for action, wildlife, and sports photography. Their zoom lenses can frequently have a much slower (smaller) effective wide-open aperture (f-stop) than DSLR or prosumer cameras, especially at the telephoto end, which further limits their utility in situations involving low light levels and moving subjects.

See also

• digital photography

• Cameras by type
• Digital photography