The reflex design scheme is a major difference between a DSLR and an ordinary digital camera (digicam), which typically exposes the sensor constantly to the light projected by the lens, allowing the camera’s screen to be used as an electronic viewfinder.
In contrast, the mirror arrangement in a DSLR usually precludes the ability to view the scene on the liquid crystal display (LCD) before the photograph is taken. However, many newer DSLR models feature live preview, allowing the LCD to be used as a viewfinder in the same way as a normal digicam, although with certain limitations and with the optical viewfinder disabled.
In most other respects, a DSLR is similar in principle and operation to a standard (non-SLR) digital camera.
[edit] DSLR design principles
1 – 4-element lens
2 – Reflex mirror
3 – Focal-plane shutter
4 – Sensor
5 – Matte focusing screen
6 – Condenser lens
7 – Pentaprism
8 – Eyepiece
A camera based on the single-lens reflex (SLR) principle uses a mirror to show in a viewfinder the image that will be captured. The cross-section (side-view) of the optical components of an SLR shows how the light passes through the lens assembly (1), is reflected into the pentaprism by the reflex mirror (which must be at an exact 45 degree angle) (2) and is projected on the matte focusing screen (5). Via a condensing lens (6) and internal reflections in the roof pentaprism (7) the image is projected through the eyepiece (8) to the photographer’s eye. Focusing is either automatic, activated by pressing half-way on the shutter release or a dedicated AF button, as is mainly the case with an autofocusing film SLR; or manual, where the photographer manually focuses the lens by turning a lens ring on the lens barrel. When an image is photographed, 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 sensor (4), after which actions, the shutter closes, the mirror returns to a critical 45 degree angle, and the diaphragm reopens and the built in drive mechanism retensions the shutter for the next exposure. There is often a ring of soft material around the focusing screen, which helps to both cushion the impact of the mirror slapping up and help seal the mirror box from light entering through the eye piece.[1] Some high end cameras incorporate a shutter into the eyepiece to further eliminate light that may enter there during long exposures.
[edit] Fast phase-detection autofocus
The diagram shown here is an over-simplification in that it omits the sensors used to activate the drive for the autofocus system. Those sensors reside at the bottom of the mirror box. In such a system, the main mirror is slightly translucent in the center, which allows light to pass through it to a secondary mirror which reflects light to the sensors below.
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 need to use the slower contrast method of autofocus.
[edit] DSLR optical viewfinder vs. digicam LCD
Depending on the viewing position of the reflex mirror (down or up), the light from the scene can only reach either the viewfinder or the sensor. Therefore, most DSLRs do not currently provide “live preview” (allowing focusing, framing, and depth-of-field preview using the display), a facility that is always available on digicams.
The advantages of an optical viewfinder are that it alleviates eye-strain sometimes caused by electronic view finders (EVF), and that it constantly shows (except during the time for the sensor to be exposed) the exact image that will be exposed because its light is routed directly from the lens itself. Compared to ordinary digital cameras with their LCDs and/or electronic viewfinders the advantage is that there is no time lag in the image; it is always correct as it is being “updated” at the speed of light. This is important for action and/or sports photography, or any other situation where the subject or the camera is moving too quickly. Furthermore, the “resolution” of the viewed image is much better than that provided by an LCD or an electronic viewfinder, which can be important if manual focusing is desired for precise focusing, as would be the case in macro photography and “micro-photography” (with a microscope).
Compared to some low cost cameras that provide an optical viewfinder that uses a small auxiliary lens, the DSLR design has the advantage of being parallax-free; that is, it never provides an off-axis view.
A disadvantage of the DSLR optical viewfinder system is that it prevents the possibility of using the LCD for viewing and composing the picture before taking it. Some people prefer to compose pictures on the display – for them this has become the natural way to use a camera. Electronic viewfinders may also provide a brighter display in low light situations, as the picture can be electronically amplified; conversely, LCDs can be difficult to see in very bright sunlight.
[edit] DSLRs with live preview
A fairly recent development in DSLRs is the increased availability of live preview options, which make it possible to use either the optical viewfinder or the LCD when composing the picture (but not both). This can be an advantage because some people simply prefer to use the display and because in some situations it is not convenient or possible to hold the camera up to one’s face to look through the viewfinder. Underwater photography, where the camera is enclosed in a plastic waterproof case, is an example of a situation where composing on the display is preferred.
Olympus introduced the first DSLR with live preview – albeit an atypical design with a fixed lens – the Olympus E-10, in the summer of 2000. Since then other manufacturers have launched DSLR models with live preview, and it is likely that other new DSLRs will eventually have this feature.
As of early 2008, some DSLRs from Canon, Nikon, Olympus, Panasonic, Leica, Pentax, Samsung and Sony all provide continuous live preview as an option. Additionally, the Fujifilm FinePix S5 Pro[2] offers 30 seconds of live preview.
Some live preview systems make use of the primary sensor to provide the image on the LCD (which is the way all non-DSLR digicams work), and some systems use a secondary sensor. Possible advantages of using a secondary sensor for live preview is to avoid additional noise that might result from the primary sensor heating up from continuous use, and allowing faster auto-focus.[3]
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.[4]
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