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 photographer can see the motive before taking an image by the mirror.
When taking an image the mirror will swing up and light will go to the sensor
instead.
1. Camera lens
2. Reflex mirror
3.
Focal-plane
shutter
4.
Image
sensor
5. Matte focusing
screen
6. Condenser lens
7.
Pentaprism/pentamirror
8. Viewfinder eyepiece
The Design of DSLR Cameras
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 microprocessors for processing the digital image, performing data storage and/or driving an electronic display.
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 Cameras
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
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.
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.
Interchangeable lenses
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 which offer a movie
mode capable of recording high definition motion video. A DSLR with this
feature is often known as an HDSLR or DSLR video shooter. The first DSLR
introduced with an HD movie mode, the Nikon
D90, captures video at 720p24
(1280x720 resolution at 24 frame/s).
Other early HDSLRs capture video using a nonstandard video resolution or frame
rate. For example, the Pentax K-7 uses a nonstandard resolution of
1536×1024, which matches the imager's 3:2 aspect ratio. The Canon EOS 500D (Rebel T1i) uses a nonstandard frame
rate of 20 frame/s at 1080p, along with a more conventional 720p30 format.
In general, HDSLRs use the full imager area to capture HD video,
though not all pixels (causing video artifacts to some degree). 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 ratio of
active pixels (to total pixels) is more susceptible to aliasing artifacts (such
as moire patterns) in scenes with particular textures, and CMOS rolling shutter
tends to be more severe. Furthermore, due to the DSLR's optical construction,
HDSLRs typically lack one or more video functions found on standard dedicated
camcorders, such as autofocus while shooting, powered zoom, and an electronic
viewfinder/preview. 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 location shooting.
Video functionality has continued to improve since the
introduction of the HDSLR, including higher video resolution (such as 1080p24) and video bitrate, improved
automatic control (autofocus) and manual exposure control, and support for
formats compatible with high-definition
television broadcast, Blu-ray disc mastering or Digital
Cinema Initiatives(DCI). The Canon
EOS 5D Mark II (with the release
of firmware version 2.0.3/2.0.4) and Panasonic
Lumix GH1 were the first HDSLRs
to offer broadcast compliant 1080p24 video, and since then the list of models
with comparable functionality has grown considerably.
The rapid maturation of HDSLR cameras has sparked a revolution
in digital filmmaking, and the "Shot On DSLR" badge is a quickly
growing phrase among independent filmmakers. Canon's North American TV
advertisements featuring the Rebel
T1i have been shot using the T1i
itself. An increased number of films, documentaries, television shows, and
other productions are utilizing the quickly improving features. One such
project was Canon's "Story Beyond the Still" contest that asked
filmmakers to collectively shoot a short film in 8 chapters, with each chapter
being shot in only a couple of weeks and a winner was determined for each
chapter, afterward the winners collaborated to shoot the final chapter of the
story. Due to the affordability and convenient size of HDSLRs compared to professional
movie cameras, The Avengers used five Canon EOS 5D Mark II and two Canon 7D to shoot the scenes from various
vantage angles throughout the set and reduced the number of reshoots of complex
action scenes.
Sony ECM-CG50 shotgun-type microphone for DSLR video capture
Manufacturers have sold optional accessories to optimize a DSLR
camera as a video camera, such as a shotgun-type microphone, and an External EVF with
1.2 million pixels.
Live preview
Nikon D90 in Liveview mode also usable for 720p HD video
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.
In 2000, Olympus introduced the Olympus E-10, the first DSLR with live
preview – albeit with an atypical fixed lens design. 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.
Larger sensor sizes and better image quality
Drawing showing the relative sizes of sensors used in current digital cameras.
Image sensors used in DSLRs come in a range of sizes. The very
largest are the ones used in "medium format" cameras, typically via a
"digital back" which can be used as an alternative to a film back.
Because of the manufacturing costs of these large sensors the price of these
cameras is typically over $20,000 as of December 2007
"Full-frame" is the same size as 35 mm film (135
film, image format 24×36 mm); these sensors are used in DSLRs such as theCanon EOS-1D X and 5D
Mark III, and the Nikon D800, D4, D610,
and Df. Most modern DSLRs use a
smaller sensor that is APS-C sized, which is approximately 22×15 mm,
slightly smaller than the size of an APS-C film frame, or about 40% of the area
of a full-frame sensor. Other sensor sizes found in DSLRs include the Four Thirds System sensor at 26% of full frame, APS-H
sensors (used, for example, in the Canon
EOS-1D Mark III) at around 61% of full frame, and the original Foveon X3 sensor at 33% of full frame (although
as of 2013, current Foveon sensors are APS-C sized).
Leica offers an
"S-System" DSLR with a 30×45 mm array containing 37 million
pixels. This sensor is 56% larger
than a full-frame sensor.
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