Birding Binocular Basics and Buying Guide

Lots of optics. Photo credit: Susan Joa.

Choosing a pair of binoculars [1] can be a daunting task. A quick search of amazon.com will deliver over 86,000 results. In this post, I hope to help you make sense of the numbers and terms you will encounter when trying to decide on which binos are right for you. I will also address some basic techniques for using your new binos that should help to cut down on the frustration associated with zeroing in on a tiny moving target you've spotted with the naked eye.

Let's take a look at the basic way binoculars and other optics are described. The focus wheel cap on the binos pictured below tells us some important information.

These binos are 10 power with an objective lens diameter of 42mm. The field of view at 1000 yards is 340 feet [a].

The first number (in this case "10") which is followed by an "X" tells us the magnification power of the binoculars. A "10 power" or "10x" bino magnifies the image ten times. As with many things in life, more is not always better. Binoculars don't just magnify rare and furtive warblers; they also magnify every little movement and shake of your hands. The higher the magnification, the more unsteady the image will be. 10 power binoculars can be very difficult to steady but a few little tricks can make them manageable [2].

David Sibley (of The Sibley Field Guide to Birds) reportedly uses 7 power as his go-to magnification. I personally prefer 10 power because I am often looking at high-altitude raptors or at shorebirds across an expanse of water. After ascending a steep learning curve, I've learned to stabilize these binos so that I can also effectively use them at closer distances. 8 power binoculars are an excellent compromise between magnification and steadiness and I recommend them for anyone looking to buy a first pair of "real" binos.

The number following the magnification power is the objective lens [3] diameter, in this case 42mm. The larger this number is, the larger the objective lens. Taken together, the magnification and the objective lens diameter will tell us something about how "bright" the view through the binoculars will be. This is determined by dividing the objective lens diameter (42mm) by the magnification power (10) giving us a value which is called the "exit pupil" (42/10=4.2). This exit pupil is measured in millimeters and directly corresponds to the diameter of the pupil in your eye. A healthy adult's average pupil size ranges from 2-4mm in bright light and 4-8mm in low light. As calculated above, my binoculars have an exit pupil of 4.2mm. This means that in normal daylight, the view through the binos will be as bright as with the naked eye. In the lower light that occurs under a heavy forest canopy or in the last hours of daylight, my binoculars will provide a reasonably bright view.

To make all of this less unclear [4], let's consider the view that would be obtained through a pair of 12x20mm binoculars. Those who aren't familiar with the ideas discussed in this post are at risk for buying such a nearly useless pair of binos. They understandably assume that more magnification is better (hence the 12x) and are looking for the lightest, most compact pair they can find (hence the small 20mm diameter). If they are able to look through these binos in a store, they may not notice how unsteady the image is because the context is quite different from trying to pick out field marks on a 3" bird in a tangle of bushes. The bright fluorescent lighting in most stores will make it less likely for the unwary shopper to notice how dim the view is. If we calculate the exit pupil on this hypothetical pair of binos (20/12=1.67) we see that the diameter is even smaller than the smallest diameter our pupils typically reach even in bright light. This means that in even slightly dim lighting when our pupils are likely to be around 4mm, all of the light coming through our binos is squeezed into a tiny little circle less than half the diameter of our light-hungry pupils. This is like trying to drink a milkshake through a coffee stirrer. The image will be so dim as to be basically unusable in anything but scorching bright light. Even in such bright light, the outrageous 12x magnification [5] will make it all but impossible to hold the image steady. In case I have completely flubbed my attempt at explaining this, consider the following diagram:

I'm saying that Nikon makes excellent products and you should all buy them. That should make it "ok" that I stole this diagram from them. 

Having thoroughly confused the issue and disgusted the audience, let's proceed to other features of a good pair of binoculars. Adjustable eye-cups are a must have in any serious pair of binos. They can be dialed-in to give proper eye-relief [6] if you need to press them right up against your glasses or sunglasses. Conversely, if you aren't wearing glasses, they can be dialed-out or anywhere in between to give the right eye-relief for you. Below, my binoculars are pictured with one eye cup dialed-in and one dialed-out to illustrate this point:

Adjustable eye cups =  adjustable eye relief. 

Lens coatings are another important factor in image quality. They can often be a large part of the price of a good pair of binoculars. As we all know, glass is reflective! Light that reflects off of your binoculars' lenses goes out into the unknown and does not reach your eye. High quality lens coatings prevent or reduce both reflection and refraction [7] and help make sure that as much light as possible reaches your eyes. All other things being equal, a more expensive pair of binoculars will have a much brighter and clearer view due in large part to lens coatings [8].

Congratulations if you've made it this far! It's almost over. If you're in the market for a set of binos for backyard bird watching or birding [9] I highly recommend a pair of 8x42 binoculars made by a company you've heard of (Nikon, Bushnell, Celestron, Vortex, etc) in the $100-$200 range. That configuration for that kind of money will get you a very good pair of binos that will stand up to some "field use" for years to come. Spending additional money will be worth it both in terms of image quality and durability. Stay tuned for the next installment of "Overcomplicating Simple Things" wherein we'll discuss how to actually use binoculars.
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1: Technically the correct usage is "binocular" as a singular word (eg. "Hand me my binocular."). However, considering several other conventions of the English language that go unchallenged ("pair of pants" etc), I'm sticking with "binoculars" or "binos" for short.

2: Even you can learn how to hold binoculars steady! Start by keeping your elbows tucked in, close together, and rest them on your chest. This creates a more stable, tripod-like, platform. Wearing a hat is a great idea for a number of reasons that are beyond the scope of this discussion but using a few fingers from your left hand to pinch the brim of the hat to the binoculars is another way to keep them nice and steady.

3: The lens you put up to your eyes is called the "ocular". The lens at the other end is called the "objective".

4: In retrospect this paragraph actually makes it more unclear.

5: 12x magnification is outrageous in the context of birding. There are many excellent pairs of 12 or even 20 power binoculars that are *intended for use with a tripod* and are sold to astronomy and big game hunting enthusiasts. Freehanding 12s is not something that's worth your effort.

6: Eye-relief is the distance between the ocular lens and your eye. If the lens is too close or too far away you will not get a complete, clear picture.

7: You've been very patient so we're not going to wade into reflection vs refraction here.

8: My pair of 10x42 Bushnell Legends is much brighter than my 10x42 Nikon Actions. A pair of 10x42 Swarovskis (at nearly ten times the price) will be significantly brighter than either of the former.

9: The former is an idle pastime. The latter is a pathological obsession.

a: Field of view is the horizontal length that is seen through the binoculars from 1000 yards away. Imagine looking 1000 yards across a field at a picket fence. If the fence is 400 feet long, you might be able to see all 400 feet of it. Because my binoculars have a field of view of 340 feet at 1000 yards, I will only be able to see 340 feet of the fence. Capiche?