Digital single-lens reflex camera

A digital single-lens reflex camera (digital SLR or DSLR) is a digital camera that uses a mechanical mirror system and pentaprism to direct light from the lens to an optical viewfinder on the back of the camera.

The basic operation of a DSLR is as follows: for viewing purposes, the mirror reflects the light coming through the attached lens upwards at a 90 degree angle. It is then reflected three times by the roof pentaprism, rectifying it for the photographer's eye. (Note that the diagram below incorrectly shows a non-roof pentaprism.) During exposure, the mirror assembly swings upward, the aperture narrows (if stopped down, or set smaller than wide open), and a shutter opens, allowing the lens to project light onto the image sensor. A second shutter then covers the sensor, ending the exposure, and the mirror lowers while the shutter resets. The period that the mirror is flipped up is referred to as "viewfinder blackout". A fast-acting mirror and shutter is preferred so as to not delay an action photo.

All of this happens automatically over a period of milliseconds, with cameras designed to do this 3–10 times per second.

DSLRs are often preferred by professional still photographers because they allow an accurate preview of framing close to the moment of exposure, and because DSLRs allow the user to choose from a variety of interchangeable lenses. Most DSLRs also have a function that allows accurate preview of depth of field.

Many professionals also prefer DSLRs for their larger sensors compared to most compact digitals. DSLRs have sensors which are generally closer in size to the traditional film formats that many current professionals started out using. These large sensors allow for similar depths of field and picture angle to film formats, as well as their comparatively high signal to noise ratio.

The term DSLR generally refers to cameras that resemble 35 mm format cameras, although some medium format cameras are technically DSLRs.

High definition DSLRs (HDSLRs)

Introduced in 2008, HDSLRs are DSLRs which, in addition to taking still photographs, offer a movie mode capable of recording high definition motion video. This feature parallels the evolution of compact digital cameras, many of which also offer HD movie mode. The first HDSLR, the Nikon D90, captures video at 720p24 (1280x720 resolution at 24 fps) using an APS-sized sensor. The Canon EOS 5D Mark II captures video at 1080p30 (1920x1080 resolution at 30 fps), as well as 1080p24 (1920x1080 resolution at 24 fps) using a full frame 35mm CMOS sensor. The 720/24p of the Nikon D90 is not a compliant frame rate for high-definition television broadcast, Blu-ray disc mastering[5] or digital cinema initiatives (DCI). The first HDSLR to shoot a standard HD broadcast, Blu-ray and digital cinema format is the Panasonic Lumix GH1 (both 1920x1080/23.976p and 1280x720/59.94p). With the release of the prosumer model Canon 7D and the consumer model Canon 550D (Rebel T2i), there are now four HDSLRs that can shoot in these standard/broadcast compliant resolutions and frame rates, with the Canon EOS 5D Mark II adding 24p and industry compliance with the release of firmware version 2.0.3/2.0.4.[2]

Less than a year after the introduction of the first HDSLR, "HD movie mode" was incorporated into entry-level DSLR camera models, the first being the Canon EOS 500D (Rebel T1i) and Nikon D5000. The 500D supports both 720p30 and a limited 1080p mode which captures 20 fps. The D5000's movie mode is comparable to the D90, with a maximum capture mode of 720p24. These entry-level cameras also use non-standard resolution and frame rate combinations.