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How does a rangefinder work? - December 1996

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I was reading my Cabela's catalog last night when I came across something that I thought looked like an interesting project. Cabela's makes a lot of hunting and fishing equipment.

One of the items available is a "rangefinder." When you are hunting animals, you need to know how far away they are so that you can figure out where you need to aim - a bullet doesn't travel in a straight line, but drops as it flies along. You have to allow for the drop when you aim, so you need to know how far away thetarget is.

Here's what the catalog says: "These particular rangefinders are quick and easy to use. You simply merge the two images into one to get exact distances. Delivers precise instantaneous distance readings."

From looking at the picture, here's what I can figure out. These things are sort of like binoculars. They are about 10" long. They have two windows on either end that face the animal. You look through the viewfinder on the back with one eye. You see two pictures of the animal. You turn something until the two images are on top of each other. It tells you how far away the animal is.

How does this thing work? How can it figure out how far away the animal is?

One thing that caught my eye is that under one of the specific descriptions it said that one particular model is accurate within 22 yards at 400 yards. That doesn't seem too accurate to me! Why wouldn't it be more accurate? What would you have to do to make it more accurate?


- Annie Fetter

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9 December 1996