Some words with a single definition actually describe a wide range of products. When it comes to optics, “rangefinder” is such a word. The word, itself, is self-explanatory, but there are many types of optical instruments that have rangefinders.
In it’s simplest form, a rangefinder is an instrument that does one and only one thing – determine distance to a selected target. The modern handheld rangefinder is a laser rangefinder; it incorporates an invisible laser that hits the target, then sensors in the rangefinder pick up the reflections of the laser beam off the target and, finally, a computer chip in the rangefinder notes the time it takes to receive the reflection and does the math. You see the distance displayed on a screen in an instant. The laser rangefinder is also incorporated into many weapons systems, surveying equipment and other high tech instruments. However, this technology is also appearing in many sporting products, such as target and hunting riflescopes, such as the Nikon Laser IRT riflescope.
That’s a far cry from the old mechanical optical rangefinders. In this type of handheld rangefinder, you see two images superimposed on each other. You then merge the images into one single image and read the distance on a dial. These handheld optical rangefinders have all but disappeared, but that system still can be found in rangefinder cameras, such as the Leica M9 camera, which is one of the very few manually focusing cameras in production.
Lastly, there are rangefinder binoculars that get you a binocular and rangefinder in one package. However, you must exercise caution, here. Only binoculars advertised as laser rangefinder binoculars use a laser rangefinder and these typically start in price with the Leupold RXB-IV . Less expensive binoculars that advertise a rangefinder do not include a laser. Instead, you see a grid of lines when you look though the binocular. You then bracket the object you wish to see between two marks on the grid, do some calculations and determine a distance. However, the one caveat with this system is that you must know the height of the object in question to do that math. This is an old technology and, for most users, not very useful. It is still offered in many marine binoculars as much out of tradition than out of practicality.