In this article we look at the reflex mirror, providing a guide to what it is for and how it works

The reflex mirror has been around for centuries, but it really came into my own in the early 20th century. Although it is best known for being at the heart of single-lens reflex (SLR) cameras, the reflex mirror was also a fundamental part of twin-lens reflex (TLR) cameras.

The arrangement of a mirror placed behind a lens in order to project the light from the lens for the convenience of viewing has been used since the 1600s in a camera obscura. Camera makers eventually miniaturised reflex mirror designs and the era of the TLR began in the 1880s, when such cameras became small enough to be handheld with the photographer viewing downwards at the waist-level viewfinder, where a view of the scene was projected by the top of two lenses. Underneath was the second lens that would expose the film.

However, it was the SLR camera that really propelled the success of the reflex mirror, with the first 35mm SLRs developed in the mid-1930s. The idea was similar to a TLR, except the camera only had one lens and the mirror was placed between the lens and the film plane.

The problem of the mirror being in the way of the film was solved in a couple of ways. In early designs the photographer had to manually lift the mirror before making the exposure. In later smaller cameras the mirror, hinged at the top, was spring-loaded and connected to the shutter- release mechanism and it flipped up before the shutter fired. But in early models the mirror had to be manually wound back down before you could take another shot. Eventually, we had the ‘quick- return’ mirror, which flipped back down as soon as the shutter had closed. However, this could not be operated particularly fast and it was quite loud.

To address these issues, fixed semi-silvered reflex mirrors were developed, also known as pellicles. Canon, in particular, made a number of pellicle mirror SLRs. Recently, Sony revived the semi-silvered mirror with its single lens translucent (SLT) alternative to the contemporary DSLR.