Binoculars (aka Field Glasses): What you Need & When you Need it
What are they?
Essentially, binoculars are two low-powered, matching telescopes yoked together. The most significant advantage over a telescope is that the twinned devices are offset from each other, just like our eyes, giving us two-eyed or binocular vision. This allows us to have depth perception, just as we are accustomed to in everyday life.
Image adapted from https://upload.wikimedia.org/wikipedia/commons/d/db/Binocular-optics.png
This is measured in powers (written as x) and is expressed as a number describing how much closer something appears to be when viewed through a particular combination of lenses. 2x would look twice as near as what you would see with the naked eye, and 8x would appear eight times closer.
Magnification can be both good and bad. It can help us see better, but it can also compress and amplify undesirable effects, canceling any gains we may have achieved.
For example, you could probably balance the end of a broom-handle on your fingertip, with the bristles much higher than the top of your head. Most of us have done this at one point. Yet, even with “rock-steady” hands, it is virtually impossible to do it with a pencil on its sharpened point. Why?
Your heart makes your entire body jiggle with every beat. This can be seen while trying to hold your arm still; it still pulses up and down, synchronously with your pulse. The pencil’s balance is so sensitive that if it becomes off-center by more than 1/10000th of the width of an atom, it begins to topple and becomes unrecoverable in less than one second.
That jiggle is enough to keep you from ever achieving balance for more than a second or two. The pencil doesn’t possess enough inertia for you to ever catch up with your own body’s jiggles.
Brooms do work because of their height, and the extra time you have to react, but compressing all those effects into the length of a pencil magnifies them to be so great that you cannot control them. With utter concentration and focus, the world record is ~12 seconds, attributed to luck as much as skill.
For that reason, with your body’s permanent jiggle, combined with our inability ever to be genuinely "still," hand-held binoculars are seldom more powerful than 15x, and typically run between 4x and 10x.
What are they Good For?
You do not use a telescope to watch a basketball game, or a microscope to watch TV—some things do not make any sense—and both of these instruments have extreme magnification with a tiny Field of View (FOV). You could never understand what you were watching. A small FOV is worse than ineffective—it makes it more challenging to acquire the information you need—like trying to look at a construction site by looking through a knothole in the surrounding fencing.
Although you could read a book from across the room with binoculars, turning the pages would be a huge hassle, so reading glasses make much more sense. You’ll find the binoculars in the 4x-8x range could be a good choice for an event on a large sports field, remembering that the bigger the area, the lower the magnification you want. For a theater stage, which is comparatively small from the highest and farthest balcony, 8x or even 10x might be called for. The required magnification is very situational dependent.
Surprisingly, the two outer binoculars have identical magnifications. The left one is for low-light situations, and the right is for daylight.
No, we’re not talking about just collecting a few berries and a small amount of wood for the campfire. We’re talking about the ability of the binoculars to collect light from the item that you wish to view…
For an outdoor sporting event in the daytime, a binocular with a 25 mm objective lens would probably be ideal. Your eye’s pupil, which controls how much light enters the eye, would likely average 5 mm in diameter during the daytime, giving it an area of about 20 mm2.
The 25 mm objective lens has an area of 491 mm2. That is a ratio of 24.6:1 compared to your eye, which is plenty of light in the daytime. Your eye's pupil will probably shrink because it is too bright, and a 50 mm lens would provide 1,960 times as much area, or 98 times the brightness—uncomfortable and entirely unnecessary for the task.
In the darkness, however, a large objective lens produces an amazingly clear image. You would be able to see so well that you could identify those raccoons raiding your neighbor’s trash in a police line-up tomorrow!
Sportsmen and women might be inclined to use a spotting scope [insert link to Spotting Scopes article here] from a hunting blind (aka “hide”), but those on-the-move often prefer a handy pair of binoculars, not only for the reduced size and lighter weight but for the lower magnification suitable for a glance without setting up a steady mount.
Birders, for example, have been known to use a clever trick. They frequently walk with a quarterstaff to help navigate rough terrain, and they also often use binoculars rated at 10x or even 15x to find elusive small birds. Higher power binoculars need to be held very steady, so they use the top of their quarterstaff as a makeshift brace for the binoculars. Damned clever, those birders!
Binoculars even have their place as low-powered astronomical instruments. Many binoculars now come equipped with a camera tripod mounting fixture right from the manufacturer. Their chief advantage is that they possess a much broader FOV, meaning you can more easily scan the sky to locate your target object instead of struggling to set up a somewhat clumsier telescope with just a tiny field of view.
It is the best way to find a comet—they're generally so faint as to be almost invisible to the naked eye. If someone tells you that there is one in a rather large constellation like Pegasus, binoculars can help you spot it in just a moment, rather than tediously scanning every bit of the constellation with a telescope.
Coatings & Optics
Modern binocular lenses are coated. Those used for non-astronomical purposes will often have a ruby coating, which tends to decrease glare and to enhance contrast. However, this tends to add a bluish-green cast to astronomical images, and also decreases the coloration of stars that tend to red part of the spectrum.
If you're only using your binoculars for siting your telescope, then this makes no difference. If it's going to be your main viewing instrument, then you might be better off to acquire a pair with a multi-coated lens. These generally have a light blue or greenish color to them from reflected light, or perhaps a variegated rainbow hue. These help with color aberration and contrast.
Binoculars are the all-round useful tool in the optics game. They have a wide field of view, and relatively low magnification, making them ideal for general use. When something more (or less) powerful is required, you have options, but for everyday purposes, you'll seldom go wrong by having a pair of binoculars handy!