DSLR design principles
Cross-section view of DSLR system:
- 4-element lens
- Reflex mirror
- Focal-plane shutter
- Image sensor
- Matte focusing screen
- Condenser lens
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 the 45 degree angle, the diaphragm reopens, and the built in drive mechanism re-tensions 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 helps seal the mirror box from light entering through the eye piece. Some high end cameras incorporate a shutter into the eyepiece to further eliminate light that may enter there during long exposures.
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 must use contrast-detect autofocus instead, which is slower in some implementations.
DSLR optical viewfinder vs. digital point-and-shoot camera 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, many older DSLRs do not provide “live preview” (allowing focusing, framing, and depth-of-field preview using the display), a facility that is always available on digicams although today most DSLRs offer live view.
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 while it is used 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 de-facto 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.