When discussing rangefinders, and how well they work, you may have heard the term Beam Divergence. What is Beam Divergence? Why is that something you need to consider when you’re out shopping for a shiny new laser rangefinder? We’re going to go over what beam divergence is, how to do the math to see what size the beam will be at a given distance, and some common numbers for rangefinders on the market. In essence the Beam Divergence is a way of measuring the size of the beam at a given distance, very similar to how you measure and rank group sizes. There are other factors that determine the level of performance you will get out of a laser rangefinder, and we’ll discuss a few of those as well!
By now you should be familiar with the concept of MOA (Minute of Angle) if you’re a reader of AccuracyTech. Minute of angle meaning where 1 MOA refers to roughly 1″ at 100 yards, 5″ at 500 yards, and 10″ at 1000 yards. The Milliradian system, MIL, works the same way with slightly different math. 1 MIL at 100 yards is 3.6″, at 500 yards it is 18″ wide, and at 1000 yards 1 MIL measures 36″ wide. Most manufacturers of laser range finders, if they publish the beam divergence at all, will publish the beam divergence using the MIL system. So if you look at the specifications of a laser rangefinder and the beam divergence is listed at 0.4mrad x 2.5mrad (like the Vectronix Terrapin), they are using the MIL system. MRAD is just another abbreviation for Milliradian. So let’s talk about that for a minute.
What that means, is that if I shoot the laser at a target that is 100 yards away the beam will measure 1.44″ x 8.64″ in size. A rectangle that is wide and short. If I shoot it at a target that’s located at 500 yards distance from the point where I’m holding the rangefinder the beam will measure 7.2″ x 43.2″ in size. Lastly, at 1000 yards the beam will be 14.4″ x 86.4″ in size. Now let’s look at what’s happening here. Obviously, the farther the target is located from the rangefinder, the wider the beam becomes over time. Why is that significant? Think about the size of the object you’re trying to range. A full size IPSC steel target is approximately 18″ x 30″ in size. At a 1000 yards, the beam covers almost the entire target vertically, which isn’t so bad. However, the beam is almost three times as wide as the target.
Target Area Clutter
You have to consider, particularly in dynamic environments, the amount of clutter in the target area. If I want to range a steel target, I fire the laser at the target. What happens if you get a sneaky match director that likes throwing the competitors a curve ball? If they put the target on the crest of a hill, with a larger hill or the side of a mountain behind it? It’s entirely possible that when you fire the laser at the target, the laser rangefinder may read the distance to the hill behind the target you want to range. The size of the beam comes into play the farther out you go because the bigger the beam…the larger the list of objects that can get hit by the beam at the same time. This is the essence of the beam divergence topic. Ideally, you want the smallest, tightest beam you can get for the most accurate return.
However, consider that you may be trying to range a small target at extended range. Think about how hard it would be to hit a full size 18″ x 30″ IPSC target with a rifle, standing, at 1000 yards. It’s easier to do with a laser because the beam isn’t affected by wind or gravity. Unfortunately, if you happen to be shivering because it’s cold, that movement affects how accurately you can hit the target with the laser just as it would with a bullet. Now take into account the beam divergence at whatever distance, and compound that by adding in a less than stable platform to fire the laser from and you begin to understand the problem. It’s easy to make a mistake like this and the match directors know it and exploit the hell out of it all the time. So what can you do about it?
Stacking The Odds
For starters you want the odds in your favor, so stack them accordingly. Buy the best laser you can afford because in the long distance shooting game, in any location but on a square range, you MUST know what the range is to the target. To aid in that decision I’ve harnessed my google-fu and come up with a short list of popular rangefinders these days, and their corresponding beam divergence. I’ve also listed some prices to give you an idea how beam divergence plays into the cost of the units and the performance they are capable of.
- Leica 1600b – 0.5 x 2.5 mrad – $800
- Leica HD-B – 0.5 x 2.0mrad -$3000
- Vectronix Terrapin – 0.4 x 2.4 mrad -$2000
- Vectronix PLRF10 – 0.3 x 1.5 mrad – $3200 (Non “C” Model)
- Vectronix PLRF15 – 0.3 x 1.5 mrad – $3400 (Non “C” Model)
- Swarovski EL Range – 0.5 x 1.5 mrad – $2300 (10×32 Model)
- SilencerCO Radius – 0.5 x 1.85 mrad – $1000
- Zeiss Victory RF – 1.6 × 0.5 mrad -$2800
While beam divergence is huge, and it’s very important to consider when you’re out shopping around for a quality rangefinder, it isn’t everything. For example, look at the beam divergence numbers for the Leica 1600b and the Vectronix Terrapin. They’re almost identical, only 0.1 MIL difference in height and width of the beam. However, I’ve seen people report spotty performance with the Leica 1600b approaching 1000 yards in sunny conditions. I’ve also ranged buildings at almost 3000 yards with the Terrapin in bright daylight. So why the performance gap? There are two components that matter with the rangefinder. The quality of the emitter, that fires the laser, and that’s the component that’s going to control not just the size of the beam…but how strong it is and the shape. The receiver on the rangefinder is also important. How well does a rangefinder do at picking up the laser returns off distant objects?
None of the units above are junk, they’re all quality items. Some just have different strengths than others. They all have different performance envelops. If you want to read an excellent article about how a number of rangefinders actually performed in the real world then I recommend the Precision Rifle Blog – Ranging Performance article. The owner at PRB actually put several high dollar rangefinders through their paces in real world scenarios and measured the performance results and ranked it all. It’s an excellent read if you’re in the market for a laser rangefinder. Remember that while beam divergence is an excellent metric to use to evaluate and get a feel for how well a laser rangefinder will perform, it isn’t everything. The phrase, “You get what you pay for,” is especially true in the rangefinder world. If you have any questions or something to add, please do so below in the comments.