Fifty-Fifty Mirrors (Beam-Splitters)

Fifty-fifty mirrors are made from optically polished glass coated on one side with a microscopically thin layer of aluminium. The density of coating determines the amount of reflection which, as the term fifty-fifty implies, is designed to return 50 per cent of the light back to source and permitting 50 per cent to pass through.

Ghost effects

With the mirror placed at 45 degrees in front of the camera lens various ghost or super-imposition effects can be achieved.

This technique, while seldom employed in modern television studios, has uses for the camcorder owner needing to combine two pictures with a single camera.

With a 10 centimetre square mirror set in front of the lens, superimposition of one picture over another can be arranged by simply dimming the light on one while brightening it on the other. An example might be to show a simulated X-ray picture of bones in a human hand using photographs or illustrations. Similarly a real hand and a skeleton hand could be used.

Depth of field

The distance from the lens via the mirror remains as important as it would under normal conditions – i.e. there will be focus problems when attempting to combine distant pictures with close-ups.

For super-imposition effects, it is invariably necessary to use black backgrounds, but they should be kept as far behind the subjects as possible to prevent spilled light causing them to register.

Auto-focus

Videocameras with auto-focus may be fooled into believing that the mirror is the subject. The use of manual focus override will normally deal with this.

One of the benefits of combining two pictures in this fashion is that the results can be seen in the viewfinder or on a monitor at the time of recording.

1. Ghosts and apparitions
A semi-transparent mirror placed at 45° to the camera axis reflects the picture it sees back into the camera lens.

2. Mirrored sparks
An actor presses a switch and is ‘electrocuted’. But the sparks, which are produced by touching two electrodes connected to a welding transformer, shower out from a hole in a metal plate and are seen superimposed over the real switch, a, The electrodes, b, Fifty/fifty mirror, c, Wall mounted light switch.