A Coincidence

Well it might be one after all.

You probably wont know what one is, you might of seen one only briefly but never registered it.

The item in question is a coincidence rangefinder, used by the military from way back in the day from around 1890 right up as far as 1960’s this very simple but very effective tool has done its job with no bells or whistles at all.

You will have seen them in war movies, especially artillery or ship battles. I have made a simple guide as the science is actually mind boggling.

 

So as you can see all you do is dial in the distance and read it off, well its a bit more than that.

You see inside of the unit are 2 objective lenses, and 3 prisms, a few more lenses and some very clever math, oh and a bit of science, and maybe some alchemy for good measure…

You see as you look through the eye piece its a bit like looking through binoculars when they are too wide for your eyes. All you get is 2 images of the same, until you start to move the dial, then as if by magic, the images move closer and closer till you end up with one.

But here is the math bit, in the view finder is a square shape etched into the lens, you have to keep it straight and true. For if you don’t it will not read true, these type of range finders are also known as Parallax range finders, yes just like your modern scopes, these units are really old school.

Mine before its paint job.

Note the square shape.

 

So as you can see its an easy operation for the user to perform, and its bloody accurate to boot. Holding a +/- 1 yard deviation  over 50 yards which is actually pretty good. bearing in mind the top laser range finders are still +/- 30cm at 50 meters. so only around a third of the deviation as its grandfather.

Now I can hear you all going why carry that big thing around and yes at nearly 30 cm long its not compact by today’s standards, but it was built circa, 1980-1990. Depending on the model and the company, this was built by a USA firm called Ranging.Inc, and the only details I could find was a patent for 1980. form these 2 gents.

Inventors:
Kester, Charles E. (Fairport, NY)
Hughes, Thomas E. (Palmyra, NY)
Application Number:
05/946836
Publication Date:
08/12/1980
Filing Date:
09/28/1978.
And this was the patent description:
The finder housing contains an eyepiece, and an optical triangulation system in which one of two optical elements is pivotal relative to the other to project two images of a remote object into coincidence at the eyepiece. The adjustable element is carried by a pivotal arm, which is connected by a finely threaded screw to a manually rotatable knob so that several revolutions of the knob are required to swing the arm from one to the other of its limit positions. A measuring tape is connected at opposite ends to the knob and passes around a spaced idler spool for movement by the knob when the latter is rotated to make the images coincide. A graduated surface on the tape is viewable through an opening in the housing to indicate the linear distance of an object from the housing when the images coincide at the eyepiece. 
As I said some math, a bit of science and alchemy. hahahaah.
The unit can also allow the user to adjust the zero point on the unit. this is done at a fixed known range, and is really easy to calibrate. mark the known range on the read out dial. then look through the unit , then with a small coin or screwdriver adjust the screw till the images  line up, easy peasey. oh and on about adjustments, the screw the other near the eye piece is also for adjustment, because you are looking at 2 images the unit can if dropped or banged by accident can loose the levels of the images so one is higher or lower than the other, this screw sorts that issue out and aligns them level, all clever stuff you know.
Hard to find a model now I’m glad i bought one when I did years ago.
Even though this unit is USA designed and made, its in meters which surprised me. Most USA items are in feet and yards, sadly I shoot in yards and had to write some converts down on the back of the unit.
As you can see in the above image.
Overall this unit is nearly 40 years old and can keep up with the hotshots of today, oh one thing this unit can do is read in the dark, as long as you can see an object through the eye piece you can range it.
So Grandad still has one up on the young-uns, as only a few leading range finders can range in the dark.
This company had several models, from what I can find out, the had the 100, for home and DIY use, it even had a tape measure built into it. this the 400 meter model and they also did a 1000 meter model, for the big guns.
So if you can find one grab one, they are fun to use and work well, Ebay USA is normally the place to look, they go cheap, but you have to add shipping, and possible import duty to add.
Overall I think its great, from 19 to 400 meters and works every time.
It might just be a coincidence – or not.