The Single Lens Reflex Camera

The basic principle of the single lens reflex (SLR) camera dates back to well before the advent of photography. The idea of using a mirror to project an image from a lens on to a flat surface was employed in the camera obscura of the 17th century.

In an SLR camera, a mirror is located behind the lens and angled at 45 degrees to direct the image up to a viewfinder. This arrangement allows you to see exactly what the lens is ‘seeing’. When you take a picture, the mirror is flipped up to allow the light from the subject to pass through to the film (or sensor in a digital SLR). When the exposure finishes, the mirror drops back to the viewing position. Obviously when the mirror is up, the viewfinder is blacked out, although this will only be momentarily with shorter exposure times.

While the key advantage of an SLR is direct viewing, the reflex mirror mechanism is noisy and creates vibrations which can be an issue in some circumstances. SLRs allow for the interchanging of lenses from ultra wide-angles to supertelephotos which gives a very high degree of versatility. Irrespective of what type of lens is fitted, the SLR’s optical viewfinder always shows what will be recorded.

The digital SLR, or D-SLR, has simply replaced film with an imaging sensor and associated in-camera processing. Externally, a D-SLR looks much the same as the old film versions and the same issues exist with the noise and vibrations created by the reflex mirror. However, the benefits, primarily, of the optical viewfinder and fast autofocusing has ensured the D-SLR has been the most popular choice of digital camera for both enthusiasts and professionals for around two decades.