THE VENERABLE SLR
For many years, the single-lens-reflex (SLR) camera has been the tool of choice for pro and serious amateur photographers, and for good reason. The SLR, be it digital or 35mm, is easily handholdable, offers a host of versatile built-in features, accepts a wide range of interchangeable lenses and provides eye-level, through-the-lens (TTL) viewing.
In an SLR, light coming through the lens strikes the SLR mirror, which directs it up to the focusing screen, where the reflected image appears laterally reversed (“mirror image”) due to the mirror. DSLRs have a pentaprism viewfinder, which re-reverses the image and sends it to the viewfinder eyepiece for easy eye-level viewing. Note that some lower-end DSLRs have pentamirror viewfinders rather than pentaprisms; the pentamirrors are more compact and less costly, though a bit dimmer.
Advantages of this system include being able to see the image as it will be recorded regardless of lens focal length, no parallax (misframing with rangefinder cameras due to the viewfinder and lens “seeing” slightly different images) and relatively easy viewing in dim light.
When you press the shutter button to make an exposure, the SLR mirror flips up, out of the light path, so the light can reach the image sensor. The viewfinder image briefly blacks out while the mirror is in the up position, and the mirror’s movement causes vibrations that can reduce sharpness during long exposures at high magnifications. Many SLRs have mirror prelocks that allow you to lock the mirror in the up position after composing and focusing, then wait for vibrations to die down before making the exposure.