The camera lucida performs an optical superimposition of the subject being viewed upon the surface upon which the artist is drawing. The artist sees both scene and drawing surface simultaneously, as in a photographic double exposure. This allows the artist to duplicate key points of the scene on the drawing surface, thus aiding in the accurate rendering of perspective. At times, the artist can even trace the outlines of objects.
While on honeymoon in Italy in 1833, the photographic pioneer William Fox Talbot used a camera lucida as a sketching aid. He later recorded that it was a disappointment with his resulting efforts which encouraged him to seek a means to "cause these natural images to imprint themselves durably".
In 2001, artist David Hockney's book Secret Knowledge: Rediscovering the Lost Techniques of the Old Masters, was met with controversy. His argument, known as the Hockney-Falco thesis, is that great artists of the past, such as Ingres, Van Eyck, and Caravaggio did not work freehand, but were guided by optical devices, specifically an arrangement using a concave mirror to project real images. His evidence is based entirely on the characteristics of the paintings themselves.
The camera lucida is still available today through art-supply channels, but is not well-known or widely used.
In the simplest form of camera lucida, the artist looks down at the drawing surface through a half-silvered mirror tilted at 45 degrees. This superimposes a direct view of the drawing surface beneath, and a reflected view of a scene horizontally in front of the artist. The instrument often includes a weak negative lens, creating a virtual image of the scene at about the same distance as the drawing surface, so that both can be viewed in good focus simultaneously.
If white paper is used with the camera lucida, the superimposition of the paper with the scene tends to wash out the scene, making it difficult to view. When working with a camera lucida it is often beneficial to use black paper and to draw with a white pencil.
The camera lucida that commercial "comp" (short for comprehensive) layout artists used in the 50's through the mid to late 80's had the lenses positioned over a platform where the original reference material (like a photo) was placed. There was a bellows arrangement and two cranks that allowed you to blow up or reduce the size of your image. You would step up and place your head under a shade canopy to see your image. The image was projected upward onto a sheet of glass that was parallel to the floor, where you would place your semi translucent sheet of white layout paper and begin to draw. Comp artists relied on this machine because it allowed the artist an accurate means of drawing their clients products quickly as well as the ability to use other photos in the composition.