These are "time of flight" systems
where the round trip propagation time is measured and divided by twice
the propagagion velocity.
The oldest optical system is based on the concept of
stadia. The idea is that you can determine distance by measuring the included angle of an object of known height.
Examples:
K&E 76 0000 Alidade,
Leitz 115A transit
Another system is based on the "split image" that's made to align by
rotating a mirror. The difference in angle between the two
mirrors can be solved based on their seperation into the range.
This is the method used by "rangefinger" cameras and the military
rangefinders that look like a drain pipe as well is civilian
rangefinders like the hand held
Edscorp Field Range Finder.
Image property range finders are used in modern digital cameras.
One way these work is to look at the image contrase as the lens is set
a various focal lengths. The highest contrast image is deemed to
be the best focused.
Time of flight is used in modern digital optical range finders like the
Busshnell golf binoculars and the Laser Technology Criterion
units. These typically have one lens for the outgoing pulse of IR
light and a seperate lens to focue the light on a fast photo sensor.
