If the second number is lower than “30”—DON’T BUY IT.
The second number of the “Optics Formula” is the diameter, in millimeters, of the lenses in the ends of the binocular away from your eyes—the objective lenses. Below 30 mm in diameter and the binocular is considered a “compact” or “pocket” binocular.
So I’m telling you to eliminate from consideration all small compact or pocket binoculars?
Yes, for your first or primary binocular that’s exactly what I’m telling you.
That’s when you want a full-sized binocular. You want a good, clear, bright, sharp image under most light conditions when you’re first making your way in the optics world. You want something you can grip with both hands as you steady it while holding it up to your eyes or eyeglasses, and you want to be able to do that quickly before whatever it is that you want a closer look at takes wing or otherwise heads for the hills.
You want something with some heft when you hold it up and proudly say, “This is my new binocular.” (“Heft” is a too-often ignored optical term.)
Owing to its size a compact binocular doesn’t let in a lot of light at the objective lens, so less light will strike your eyes—giving you a less-bright image that may work for sunny days but not for those times and situations of compromised light conditions—thereby restricting when and where you’ll be able to use it. Dusk? Forget about it. Severely overcast days? Probably not. Rainforest canopy? Well, see that woman to your right? She has a 7 X 42—borrow that (and I’ll bet you buy one when you get home…that is, if you can find one!)
Also because of their small size, in many cases compacts will be harder to use and you won’t be able to grasp them firmly with both hands, leading most to adopt a “teacup”-style grip that will make it harder to hold them steady when held to the eyes, thus leading to quicker hand and arm fatigue.
For all these reasons compacts have a poor reputation among serious binocular users. Years ago I bought a compact specifically for desert hiking—an awesome Leica 8 X 25—which I proudly showed off to my supervisor at the time, arguably one of New Jersey’s best and most well-known birders. He took one glance at the small optic in my hands and blurted out, “why would anyone buy that binocular?” Yes, that looks cruel on the page (and admittedly it did bother me at the time since I had paid a fair amount for it) but it merely illustrates the degree to which good birders count on a bright image for bird viewing and identification, and for them the light provided by compacts is simply not up to the task.
But despite this conventional wisdom and the aforementioned downsides, a compact binocular does have its place—but as an additional binocular for specialized uses like hiking, kayaking, and any travel where there’s stringent restrictions on luggage weight. In those and other instances it’s clearly better to have something rather than nothing at all—and where the purchaser is well aware of the compact binocular’s many downsides before laying down his or her money.
One final point about these small binoculars, one that is a particular pet peeve of mine. Walk into many stores that sell nature optics and ask for a “children’s binocular” and more often than not the untrained salesperson (or even a trained one looking for a quick, easy sale) will suggest a compact binocular—you know, a small binocular for a small human being?
No. No. No. As difficult as it often is for an adult to use a compact, it’s usually way worse for kids. Even though their hands are smaller, a compact binocular is almost always a highly inappropriate choice for children. Leave (or hang up on) any store making that recommendation.
Besides, several optics manufacturers make a perfect binocular for kids—and that’s discussed elsewhere on this site.