DSLR design principles

A camera based on the single-lens reflex (SLR) principle uses a mirror to show the image that will be captured in a viewfinder. 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, either focused or unfocused through the eyepiece (8) to the photographer's eye. Focusing is usually 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 moves 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.

Fast phase-detection autofocus

The diagram shown here is an over-simplification in that it omits the sensors used to drive the autofocus system.

DSLRs typically use a phase detection autofocus system. This method of focus is very fast, and results in less focus "searching", but requires the insertion 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.

DSLR optical viewfinder vs. digicam LCD display

Depending on the 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 split second of 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 LCD displays 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 display or an electronic viewfinder, which can be important if manual focusing is desired for precise focusing, as would be the case in photomacrography or photomicrography (commonly referred to as "macro-photography" and "micro-photography").

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 display 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. LCD displays and electronic viewfinders may also provide a brighter display in low light situations, as the picture can be electronically amplified; conversely, LCD displays can be difficult to see in very bright sunlight.

DSLRs with live preview

A fairly recent development in DSLRs is the emergence of live preview options, which make it possible to use either the optical viewfinder or the LCD display 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 your 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, the Olympus E-330, in the spring of 2006. Since then other manufacturers have launched DSLR models with live preview, and the possibility exists that eventually all new DSLRs will have this feature.

As of the summer of 2007, the Canon EOS-1D Mark III, Canon EOS-1Ds Mark III, Canon EOS 40D, Nikon D3, Nikon D300, Olympus E-330, Olympus E-410, Olympus E-510, Panasonic Lumix DMC-L1, Panasonic Lumix DMC-L10 and Leica Digilux 3 all provide continuous live preview as an option. Additionally, the Fujifilm FinePix S5 Pro offers 30 seconds of live preview.

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.^[1]

DSLR design considerations

Pentaprism vs. penta-mirror

Some DSLR's use a pentamirror instead of the traditional pentaprism. The pentamirror design is composed mostly of plastic and is lighter and cheaper to produce. Pentaprisms provide larger and brighter images through the viewfinder.^[

Sensor size and quality

The image sensor in a DSLR is typically much larger than the one in a consumer-level digicam. A larger sensor allows better image quality, lower noise, shallower depth of field, higher sensitivity, and increased latitude and dynamic range. The SLR design allows the use of such large sensors as the main sensor is typically only exposed while the picture is being taken, in contrast to the typical 'digicam' design which uses the sensor to provide live view. Continuous operation of the large sensors required for low-noise images would result in sensor heating that could reduce image quality.^[2]

Many DSLR sensors are roughly APS-sized, that is, approximately 22 mm × 15 mm, a little smaller than the size of an APS-C film frame, much smaller than a frame of 135 film.

High-end digicams such as the Nikon Coolpix 8400 and the Coolpix 8800 have an 8.8 by 6.6 mm sensor (2/3 inch format, 11 mm diagonal), about five to seven times smaller area than a typical DSLR sensor.

Lower-end digicams have even smaller sensors, which usually, but not always, results in lower image quality than DSLRs. Some digital SLRs have sensors the same size as 35mm film; such cameras are referred to as "full-frame" cameras and are generally much more expensive. The Canon EOS 1Ds, the Canon 5D, and the Nikon D3 are examples of cameras using a full-frame sensor.