If the second number of the “optics formula” is higher than “45,” DON’T BUY IT.
Uh-oh, I think I’m in trouble again.
To review, the second number is the diameter of objective lens, or the end opposite the end in which you’re looking, expressed in millimeters. It’s the jumping-off point to how bright the binocular will be, so once again there’s a tendency to think “bigger is better” and “more light in, more light out.”
Um, yes and no.
A lot happens to that light as it journeys from its objective lens entry point, through the barrels, out the ocular lens (the eyepieces) and into your eyes, so the “more in, more out” concept is even more of an over-simplification than…well, all the other optics over-simplifications.
True, you want the brightest image possible, but yet again we find ourselves asking the question “but Mr. Science (that’s me in this scenario), at what cost?” For example, a 100 mm objective lens would give you a ton of light-gathering power, but you’d have to roll it around in a little red optics wagon to go from place to place—and that wagon’s not included with the binocular.
So the cost of a larger, greater light-gathering objective lens that would give you that brighter image is…
Weight.
When you increase the size of the objective lenses on the non-eyepiece ends of the barrels of your binocular, you are by definition increasing the amount of glass in those ends—and that’s not just any glass, that’s dense, heavy optical glass—well, that’s assuming it’s not made by Hasbro.
I talk more about weight elsewhere, but for now just know that there is almost always a trade-off between weight and brightness, and that, trust me, you’re better off if you let weight win—in most cases, choose the lighter, slightly-less bright binocular and I guarantee you that you won’t regret it.
Ask yourself: which is brighter: the less-heavy binocular that you take with you, or the greater light-gathering but heavier one you leave in the car because you don’t want to carry that “thing” around?
As with anything else, there are exceptions to the “nothing over 45” rule. If you’re a woman or man with good arm and upper body strength and/or large hands you may feel more comfortable holding on to a larger binocular—you may even feel that an optic with some “heft” to it helps you keep it steady as you hold it up to your eyes. If this describes you than by all means try out some bigger 50mm binoculars. But bear in mind the following rule which, despite the fact that it defies all laws of physics, is an accepted truth among birders and other binocular users:
“A binocular is heavier at the end of the day than it is at the beginning.”
If you can explain the physics behind that, please contact me.