Perhaps a weight suspended by a spring, with a laser rangefinder on the bottom?
From the newscientist.com
And that's just a few interchangeable-lens 35mm rangefinder cameras I can think of.
From the guardian.co.uk
Nonetheless rangefinder cameras have advantages over SLRs for certain applications.
From the en.wikipedia.org
I completely agree with the quality of imaging in Leica rangefinder cameras.
From the economist.com
Grace carried a Leica rangefinder and took personal black-and-white photos.
From the lens.blogs.nytimes.com
All these features combined create a unique GPS rangefinder that is unequaled in golf.
From the techcrunch.com
Why not use a laser rangefinder and then calculate the correction required.
From the newscientist.com
At the heart of this type of scanner is a time-of-flight laser rangefinder.
From the en.wikipedia.org
The laser rangefinder only detects the distance of one point in its direction of view.
From the en.wikipedia.org
More examples
A measuring instrument (acoustic or optical or electronic) for finding the distance of an object
A rangefinder is a device that measures distance from the observer to a target, for the purposes of surveying, determining focus in photography, or accurately aiming a weapon. ...
A rangefinder camera is a camera fitted with a rangefinder: a range-finding focusing mechanism allowing the photographer to measure the subject distance and take photographs that are in sharp focus. ...
A device included on many cameras as an aid in focusing.
This is the viewfinder on most smaller digital cameras and is a separate viewing device which is independent of the lens. It is often above and to the right or left of the lens. ...
Rangefinder is the name given to an optical device used to determine distance. It works by superimposing 2 slightly different views of the scene and allowing the user to adjust one until it aligns perfectly with the other. ...
An instrument for measuring the distance from the observer to a particular object, as for adjusting the focus of a camera. A rangefinder is included as an integral part of many cameras as a focusing aid.
Is a type of camera which has separate lenses for shooting and for looking through. The most noticeably difference from a conventional camera is that a rangefinder does not have a mirror and a prism viewfinder.
A type of viewfinder that uses one lens for focusing light onto the focal plane, and another for framing your image. Optically, a rangefinder is much less complicated, and therefore less expensive, than an SLR configuration.