A Tribute to Ageless Technologies: Using Parallax to Measure Distance

Quick. Hold up your thumb and use it to cover up a far away object.  Then close one eye at a time and watch your thumb jump from one spot to another.  Whichever eye shows your thumb correctly positioned over the object is your dominant eye.  If you are like two-thirds of the world’s population you are right-eyed. Otherwise, you are the ocular equivalent of a left hander.  While humans process images using both eyes, the brain must ultimately make a decision as to which of the overlapping fields of view to use for precise positioning.  The result is a dominant eye.

This exercise is ultimately a demonstration of parallax – the difference in apparent position of something based on alternate lines of sight.  But parallax is not just a parlor trick used to determine your dominant eye. Parallax also shows up as that annoying effect you get when trying to merge different photos into a panoramic image.  If the camera is not rotated precisely, alternate lines of sight will create parallax error when the photos are merged.  Parallax, when used purposefully, can also be a tool for measuring distance.  Parallax has been used by astronomers for hundreds of years for measuring the distance to nearby stars.  In the case of stellar measurement, the opposing lines of sight are created by two different positions of the Earth as it rotates around the Sun.

It is a sad fact of modern technology that most of the principles of operation are buried too deeply for us to see.  Despite our confident ability to master their menus and buttons, our cameras are essentially black boxes to us. Fundamental technologies are nestled into tiny integrated circuits and masked with patents and obfuscation.  These integrated circuits are then populated onto complex circuit boards and hidden inside artfully designed plastic. Dare to unscrew anything and poof – there goes your warranty.  In the end, we are left to stupidly press buttons and marvel in awe at the output of our extraordinary machines.

With this frustration in mind, it was with interest that I came across this article on how to build a homemade rangefinder. The article was from a 1935 edition of Popular Science which details with clarity and nonchalance a simple method for determining proper focus distance using only the parallax of your two eyes. There are many variations on this invention but none which have this combination of simplicity and mechanical elegance. After reading the article, I was inspired to reproduce the rangefinder and see how it works.  After a few evenings in the garage, victory was mine – the 1935 rangefinder reproduced in all its glory.  Instead of aluminum, I used cherry and brass for a more classic, Jeffersonian look.

Devices such like this work by measuring the angular line of sight between your two eyes.  For close objects, the lines of sight for your two eyes form a fairly large angle.  For far away objects, your lines of sight are virtually parallel.  This rangefinder measures the angle by forcing your lines of sight through small peep holes.  A cam lever is then used to adjust the angle until you can see the same object through both peep holes.

The video below shows the rangefinder in action.


Measuring Distance Using Parallax from Camera Technica on Vimeo.


Although the rangefinder is actually quite accurate for certain distances, it is (sadly) really only useful for nostalgic purposes.  Autofocus lenses and laser rangefinders have long since surpassed human parallax for measuring distances.   But there is something to be said for a technological principle so simple and elegant that you can hold it in your hand.  My rangefinder will hold a proud place on my mantle as a reminder that some technologies are never obsolete.

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  1. systemaddict says:

    Cool bit of kit, but how do you calibrate it?

  2. Argyll Rabbiter says:

    I have been looking for a range finder like this for ages- so thank you for posting this article. Before I go to the trouble of building one could you tell me how accurate yours is +/- at 15m, 20m, 25m,30m, 35m, 40m?

    I would be building it to range find on rabbits – if accurate enough I like the idea of building one of these as opposed to taking battery dependant lazer range finder into the filed.