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StationaryAA

How did stereoscopic and coincidence rangefinders work?

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I'm familiar with the mil system for long range shooting. (Range equals target size divided by the size seen through the scope in mils).

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https://en.wikipedia.org/wiki/Coincidence_rangefinder

wiki is your friend

 

and

http://www.kbismarck.org/forum/viewtopic.php?t=3704

goes into more detail on German (stereo) vs British (Co-incidence) (and by extension Japanese) systems

US used stereoscopic type.

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Coincidence rangefinders work by having two halves of the picture projected to a central eyepiece from off-set prisms/mirrors. You adjust the prisms/mirrors until the picture correctly lines up, at which point the range is deducted from the angle of the prisms by the rangefinder. 

 

Stereoscopic systems means that you have a binocular eyepiece, with each eye getting it's own picture from the prisms. You adjust the prisms until the pictures line-up in your head. 

 

Coincidence is simple to use, but can be fooled by various obstructions and stuff like dazzle camouflage, while stereoscopic rangefinding needs practice to get to the same accuracy, but once achieved is much more reliable. 

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Coincidence rangefinders work by having two halves of the picture projected to a central eyepiece from off-set prisms/mirrors. You adjust the prisms/mirrors until the picture correctly lines up, at which point the range is deducted from the angle of the prisms by the rangefinder. 

 

Stereoscopic systems means that you have a binocular eyepiece, with each eye getting it's own picture from the prisms. You adjust the prisms until the pictures line-up in your head. 

 

Coincidence is simple to use, but can be fooled by various obstructions and stuff like dazzle camouflage, while stereoscopic rangefinding needs practice to get to the same accuracy, but once achieved is much more reliable. 

 

That's what I found looking around but I can't seem to find how it actually works.  For example the mil system works because at very small angles the arc (the height of the object) is pretty much equal to a straight line (the opposite side of a triangle).

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That's what I found looking around but I can't seem to find how it actually works.  For example the mil system works because at very small angles the arc (the height of the object) is pretty much equal to a straight line (the opposite side of a triangle).

 

I dont think I understand you. I'll try to explain if you mean you don't understand how the range is worked out, which is sort of what I think you're asking:

 

The prisms are placed apart on two sides. You have some sort of system that angles the prisms in towards each other, or out towards being paralel to each other. Now in laymans terms, imagine each of these prisms gives out a cone or a line representing what each of these prisms can "see". The idea is that you now adjust the angle of the prisms that the two  cones or lines intersect over the target. When this is achieved, the range can be calculated via the angle at which the prisms are currently set. 

 

Coincidence_Range_Finder.jpg

Basically the two lines are adjusted by angling the prisms, and when they intersect over the target, the picture in your eyepiece will lineup correctly. 

 

This is why the width of the rangefinder is so important, as it is technically "more sensitive", as the lines are spaced further apart, so at range they still keep a significant angle on the prisms to calculate from, while a short rangefinder's prisms would be already reflecting practically perpendicularly. 

 

And to calculate the range is pretty easy with trigonometric functions, by simply halving the triangle that the two prisms and the rangefinder make towards the target - the line going from the prism to the target is the hypotenuse, the half of the rangefinder is the adjacent side and the line going perpendicularly from the adjacent is the opposite side. The angle of the prism is known, and the angle between the adjacent and opposite side is 90 degrees. 

 

So let's say we have a 9 meter wide rangefinder and the angle of the prism is 85 degrees. This means the angle between the hypotenuse and the opposite side (where they intersect is where the target is) is 5 degrees. Going of tan(85*) x 4,5m you get a range of 51,43 meters. If my math is still worth anything at least lol

Edited by chaplainDMK
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