A digital single-lens reflex camera (also called a digital SLR or DSLR) is a digital camera combining the optics and the mechanisms of a single-lens reflex camera with a digital imaging sensor, as opposed to photographic film.
The reflex design scheme is the primary difference between a DSLR and
other digital cameras. In the reflex design, light travels through the
lens, then to a mirror that alternates to send the image to either the viewfinder
or the image sensor. The alternative would be to have a viewfinder with
its own lens, hence the term "single lens" for this design. By using
only one lens, the viewfinder presents an image that will not perceptibly differ from what is captured by the camera's sensor.
The design of DSLR cameras
Cross-section view of a DSLR. Electronics and display not shown.
- Camera lens
- Reflex mirror
- Focal-plane shutter
- Image sensor
- Matte focusing screen
- Condenser lens
- Pentaprism/pentamirror
- Viewfinder eyepiece
Like SLRs DSLRs typically use interchangeable lenses (1) with a proprietary lens mount. A movable mechanical mirror system (2) is switched down (exact 45-degree angle) to direct light from the lens over a matte focusing screen (5) via a condenser lens (6) and a pentaprism/pentamirror (7) to an optical viewfinder eyepiece (8). Most of the entry level DSLRs use a pentamirror instead of the traditional pentaprism.
Focusing can be manual or automatic,
activated by pressing half-way on the shutter release or a dedicated AF
button. To take an image, the mirror swings upwards in the direction of
the arrow, the focal-plane shutter (3) opens, and the image is projected and captured on the image sensor (4),
after which actions, the shutter closes, the mirror returns to the
45-degree angle, and the built in drive mechanism re-tensions the
shutter for the next exposure.
Compared to the newer concept of mirrorless interchangeable-lens cameras this mirror/prism system is the characteristic difference providing direct, accurate optical preview with separate autofocus and exposure metering sensors. Essential parts of all digital cameras are some electronics like amplifier, analog to digital converter, image processor and other (micro-)processors for processing the digital image, performing data storage and/or driving an electronic display.
Phase-detection autofocus
Main article: Phase detection autofocus
DSLRs typically use a phase-detection autofocus system. This method
of focus is very fast, and results in less focus "searching", but
requires the incorporation of a special sensor into the optical path, so
it is usually only used in SLR designs. Digicams that use the main
sensor to create a live preview on the LCD or electronic viewfinder must
use contrast-detect autofocus instead, which is slower in some
implementations.
Features commonly seen in DSLR designs
Mode dial
Digital SLR cameras, along with most other digital cameras, generally have a mode dial
to access standard camera settings or automatic scene-mode settings.
Sometimes called a "PASM" dial, they typically provide as minimum
Program, Aperture-priority, Shutter-priority, and full Manual modes.
Scene modes vary and are inherently less customizable. They often
include full-auto, landscape, portrait, action, macro, and night modes,
among others. Professional DSLRs seldom contain automatic scene modes
because professionals understand their equipment and can quickly adjust
the settings to take the image that they want.
Dust reduction systems
Main article: Dust reduction system
A method to prevent dust entering the chamber, by using a "dust
cover" filter right behind the lens mount, was used by Sigma in its
first DSLR, the Sigma SD9, in 2002.[citation needed]
Olympus used a built-in sensor cleaning mechanism in its first DSLR that had a sensor exposed to air, the Olympus E-1, in 2003.[citation needed]
Interchangeable lenses
Main articles: Photographic lens and Lenses for SLR and DSLR cameras
The ability to exchange lenses, to select the best lens for the
current photographic need, and to allow the attachment of specialized
lenses, is one of the key factors in the popularity of DSLR cameras,
although this feature is not unique to the DSLR design and mirrorless
interchangeable lens cameras are becoming increasingly popular.
Interchangeable lenses for SLRs and DSLRs (also known as "Glass") are
built to operate correctly with a specific lens mount
that is generally unique to each brand. A photographer will often use
lenses made by the same manufacturer as the camera body (for example, Canon EF lenses on a Canon body) although there are also many independent lens manufacturers, such as Sigma, Tamron, Tokina, and Vivitar
that make lenses for a variety of different lens mounts. There are also
lens adapters that allow a lens for one lens mount to be used on a
camera body with a different lens mount but with often reduced
functionality.
Many lenses are mountable, "diaphragm-and-meter-compatible", on
modern DSLRs and on older film SLRs that use the same lens mount.
However, when lenses designed for 35 mm film or equivalently sized
digital image sensors are used on DSLRs with smaller sized sensors, the
image is effectively cropped and the lens appears to have a longer focal
length than its stated focal length. Most DSLR manufacturers have
introduced lines of lenses with image circles optimized for the smaller
sensors and focal lengths equivalent to those generally offered for
existing 35 mm mount DSLRs, mostly in the wide angle range. These lenses
tend not to be completely compatible with full frame sensors or 35 mm
film because of the smaller imaging circle and, with some Canon EF-S lenses, interfere with the reflex mirrors on full-frame bodies.
HD video capture
Since 2008, manufacturers have offered DSLRs capable of recording high-definition video.
In capturing HD video, HDSLRs use in general the full imager area,
though not all pixels, causing video artifacts to some extent. Compared
to the much smaller image sensors found in the typical camcorder, the
HDSLR's much larger sensor yields distinctly different image
characteristics.
HDSLRs can achieve much shallower depth of field and superior low-light
performance. However, the low proportion of active pixels is more
susceptible to aliasing artifacts (such as moiré patterns) and the CMOS rolling shutter
tends to be more noticeable. Furthermore, due to the DSLR's optical
construction, HDSLRs typically lack one or more video functions found on
other camcorders, such as autofocus while shooting; powered zoom; and
an electronic viewfinder. These and other handling limitations prevent
the HDSLR from being operated as a simple point-and-shoot camcorder,
instead demanding some level of planning and skill for shooting.
Video functionality has continued to improve since the introduction
of the HDSLR. Among the improvements include higher-resolution video
(such as 1080p24); improved autofocus and manual exposure control; support for formats compatible with high-definition television broadcast; Blu-ray disc mastering;
Live preview
Main article: Live preview
Early DSLRs lacked the ability to show the optical viewfinder's image on the LCD display – a feature known as live preview. Live preview is useful in situations where the camera's eye-level viewfinder cannot be used, such as underwater photography where the camera is enclosed in a plastic waterproof case.
Olympus introduced the Olympus E-10
in the northern-hemisphere summer of 2000, which was the first DSLR
with live preview – albeit an atypical design with a fixed lens. In late
2008, some DSLRs from Canon, Nikon, Olympus, Panasonic, Leica, Pentax, Samsung and Sony all provided continuous live preview as an option. Additionally, the Fujifilm FinePix S5 Pro offers 30 seconds of live preview.
On all DSLRs that offer live preview via the primary sensor, the
phase detection autofocus system does not work in the live preview mode,
and the DSLR switches to a slower contrast system commonly found in
point & shoot cameras. While even phase detection autofocus requires
contrast in the scene, strict contrast detection autofocus is limited
in its ability to find focus quickly, though it is somewhat more
accurate.
A new feature via a separate software package introduced from Breeze
Systems in October 2007, features live view from a distance. The
software package is named "DSLR Remote Pro v1.5" and enables support for
the Canon EOS 40D and 1D Mark III.
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