How to build a laser rangefinder

Camera Rangefinders are not that complicated. There are some mirrors in those things and the merging of two images which makes things a bit confusing, but explaining the basic principles of range finding aren’t difficult.

laser_diagram

Take two laser pointers. Point one straight and have another one (on the same plane) point at an angle. The point of intersection between these two is the distance to your subject. Since the laser on the right is articulating, you can map the degrees of angle the laser to corresponding distances to the subject. Here’s a nice triangle and here are it’s parts:

  1. Adjacent Side. This is the distance to the subject from the static laser.
  2. Opposite side. This is the distance between the lasers.
  3. Hypotenuse. This is the distance to the subject from the articulating laser.
  4. 90 Degree Angle.
  5. Articulating Laser Angle.

Now, how do I incorporate this into my 3D printed camera to finally make it a rangefinder!? Well, I need to get measurements of all of the above as a first pass. But some challenges:

  1. The distance stated on every lens barrel is the distance from the film plane to the subject, not the ‘rangefinder plane’ (or laser plane as I have it above). So I need to adjust for that.
  2. Interfacing the articulating laser with the lens helicoid. This requires some custom manufactured parts. 3D printing has been my friend throughout my custom camera journey so I’m going to continue to explore that as an option, but it may not have the resolution necessary for this reason…
  3. The shorter the distance between the lasers, the more ‘delicate’ the focus mechanism. In other words, a difference in +/- 0.01 degrees could throw off the intersection of the lasers and therefore the distance to the subject by centimeters. To give me more margin of error (and therefore more accuracy), I’ll need to increase the distance between the lasers.
  4. We’re using Mamiya Press lenses. To focus a 150mm lens from infinity to 3 feet requires around 20mm of travel. To do the same with a 50mm lens requires 5mm of travel. But the rangefinder mechanism doesn’t know that. Mamiya needed to have interchangeable lenses (as many rangefinders do). So to solve this problem they created a ‘focus flange’ (my name) which interfaces with the rangefinder. This has a specific travel that is universal across all lenses (approx one millimeter). I’m having a very difficult time measuring this travel with my calipers… Going to need to find another way.

Basically I need to reverse engineer a lot of things that a bunch of Japanese guys did in the 60’s. This is gonna be fun :-).

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