The Most Important Binocular Term

You can ignore most of those technical terms like “twilight factor,” “brightness index,” “chromatic aberration,” “aplanatic refraction” and the like—pay close attention to the weight.

Actually, I made up “aplanatic refraction”, but when you’re buying a binocular in order to enjoy birds and other things in nature, it’s every bit as useful at the other three.

“For the most ideal viewing of everything from backyard birds to gargoyles on the tops of European gothic cathedrals, a binocular with an aplanatic refraction ratio under 8 is strongly advised.”

It’s not that these tech terms associated with optics are bogus—all of them (well, except “aplanatic refraction”) do illustrate something about the binoculars to which they are applied. But they are largely unnecessary to your purchase decision (especially if you’re a first-time buyer) but worse, they are can be counterproductive in that they add to the confusion and the general level of “noise” surrounding optics and a buying process that could and should be simpler than it often is.

Let’s take “twilight factor” as an example—and we’re not talking about the ability of a specific cute teenager to attract the attention of a similarly cute teenage vampire.

If you’re researching optics on the internet you’re likely to see “twilight factor” among the optical performance terms bandied about. Historically it has been used as an indicator of how comparatively well a binocular will perform in low light, and you can calculate its value on any given binocular by the following formula (get out your slide rules, folks):

Multiply the magnification by the objective lens diameter (i.e. multiply the first and second numbers of the “optics formula”) and take the square root of the product. So for a 7 X 42 binocular multiply 7 X 42 (294) and take the square root of that (=17.15). So the twilight factor of a 7 X 42 binocular is 17 (rounded off).

“Wow—17? I’ll take it!”

Seriously, calculating twilight factor successfully may get you a higher SAT score but it won’t help you buy a binocular. In my many years of selling optics I have never had to refer to it during the sales process—although I was often asked about it during optics workshops by attendees who had seen the term while researching optics online.

The same is true for all those other terms and specs that do tell you something about a binocular but add little or no value to custom fitting that binocular to you.

So…what term(s) should you pay attention to? My top 3 are weight, weight and weight.

“Weight” is my favorite too-often-neglected binocular spec. How much weight you’re holding up to your eyes and around your neck makes a HUGE difference in the enjoyment of your binocular (much more than the square root of anything), and thus how often you actually take it out and use it. I’ve seen too many binoculars gathering dust, despite stellar specs, simply because they were too heavy for the user to enjoy.

On a couple of occasions, over my strong objections, customers have purchased expensive 50mm objective (second number in the formula) optics because a “friend” told them “they’re the brightest ones around,” and yes, they do look bright for the few minutes of quick inspection they undergo (I sold binoculars for a nonprofit conservation organization, so sales, even those of which I didn’t approve, helped fund the mission work of the organization). But sometime after purchase they returned, sheepishly, to buy smaller, less-heavy optics (unfortunately the number of people who bought optics against my advice, only to later regret it, could fill a large birding tour bus).

So…forget all the other terms and specs—the brighter, sharper, better binocular is the one that you bring with you and use rather than leave at home or in your car. The $300 binocular with the agreeable weight that is pleasant to use is superior to the $600 heavier binocular that you leave behind—despite what their specs tell you.

So how much should a binocular weigh? I don’t know…how much should a person weigh?

Which is to say there’s no one answer. “Light weight” full-sized (obviously most compacts will be light) binoculars will weigh from around 16 oz. to around 22 oz. Over 30 oz. and they’re going to feel heavy in the hand and up to the eyes but for some people that won’t be a problem. Around or over 3 lbs.? Well, good luck with that. 

I recommend never buying a binocular over 2 lbs. without an extended tryout first. Of course this might be a problem with the diminishing number of brick & mortar stores where optics are sold by knowledgeable salespeople, but certainly this is the preferable option.

Most reputable online optics dealers have expert customer service help and liberal return policies, so after answering all you questions they’ll send your choice (or even your top 3!) which you can try out and return if you’re not satisfied—or if you like the binocular but decide it’s too heavy.

Whatever you do, spend lots of quality time with your new binocular before you buy—if it’s the right choice you’re going to be holding and wearing it so much your friends won’t recognize you without it!