All You Need to Know About Rangefinders

Almost every bowhunter who shoots a compound bow uses a rangefinder. The general assumption is often that "a rangefinder is a rangefinder," and they are all created equal; however, that is far from the truth.

Each rangefinder has its own unique personality, flaws and limitations. Spending a lot of money on a rangefinder does not guarantee you will end up with a quality instrument. For example, some of the most expensive rangefinding binoculars I’ve tested have performed the worst under field conditions.

I’m an optics junkie and have bought and tested almost every brand of rangefinder on the market, both monocular and binocular models, as well as rangefinding bow sights. I’d like to emphasize again that in my experience, price does not necessarily equate to accuracy and performance.

I’ll spend the next few columns helping you understand how rangefinders work and encouraging you to test yours for effectiveness and accuracy. Buying the best rangefinder you can afford and becoming intimately familiar with it will make you a much more effective bowhunter.

How They Work

Laser rangefinders send a laser beam to the target to determine its distance. The laser is beamed to the target, hits the target and then reflects back to the sensor inside the rangefinder. The time it takes the laser to reflect back to the device (time of flight) is used to calculate the distance. The precision of the instrument is determined by the type of laser pulse, the speed of the receiver and the power of the beam. A rangefinder that uses very short laser pulses and has a very quick detector will range more accurately.

By definition, a laser beam is a very narrow beam of light. However, contrary to popular belief, the beam does not stay the same size; it becomes larger the further it gets from the device. This phenomenon is called divergence. The beam is also widened and distorted by the very air it travels through.

If a portion of the laser’s light reflects off a leaf or branch that is closer than the object to be ranged, the distance that is provided will be the distance to the closer object. The smaller the laser’s divergence, the less likely it is to strike objects close to, but not in, the center of the path of the beam. So, a laser with a small divergence number is less likely to give a false reading than a laser with a larger divergence number.

Some manufacturers provide a divergence number in the specifications for their devices. Unfortunately, most manufacturers do not. SIG SAUER is one of the few manufacturers that does provide this info. I assume they give it out because their rangefinders have very small divergence numbers and they are proud of that fact — and rightfully so! The divergence number is very valuable information when choosing a rangefinder. Typically, the smaller the divergence number, the better the unit's performance.

Another important piece of information is the shape of the laser beam. Some laser beams are round and some are oval shaped, and this information is provided in the divergence specifications.

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The shape of the beam influences how you use the rangefinder. We’ll discuss testing the abilities of particular rangefinders later in this series — the degree of divergence for a rangefinder can be guessed by doing a few simple tests/experiments I will outline at that time.

Lastly, the more reflective the object you are ranging, the further out the rangefinder will work. Dark, non-reflective items reflect poorly and cannot be ranged as far as more reflective surfaces. Make sure the rangefinder you choose works on black targets.

Monocular vs. Binocular

Hunting rangefinders come in monocular and binocular configurations, and I have always preferred a rangefinding binocular rather than a monocular. Binocular rangefinders provide many advantages, and they tend to have higher magnification than monocular rangefinders. They also tend to have a smaller, finer reticle (the reticle is the red, green or black square, circle or dot you place on the object to be ranged). The higher the magnification and the smaller the reticle, the more precise you can be when ranging small items, or when ranging through holes in vegetation. If you are steady enough, you can theoretically range an antler tip above the grass at 40 yards with some of the more accurate devices.

Binocular rangefinders tend to have a more powerful laser with less divergence, and they have higher magnification, so they will usually be more precise than monocular rangefinders. They also are steadier in the hand because they are larger and heavier. The downside of binocular rangefinders is that they are usually far more expensive than monocular rangefinders.

Unfortunately, some of the best binoculars (optically speaking) have rangefinders that are either not accurate at measuring distance or, more frequently, perform poorly when determining angle compensation. Often, these high-priced rangefinders are ergonomically challenging, not properly calibrated or have lousy lasers.

Check back next issue as we continue to explore this topic in greater detail.