Judging by looks alone, a monocular appears as something in between a binocular and a spotting scope. It has a similar objective diameter to a binocular, and it is usually used without a tripod, but it only allows for magnification for one eye, like the much larger scope. The relevant specifications are also fairly comparable to those of binoculars, as well as features such as the prism design or the lens coating used. It won’t be entirely off to basically call a monocular “half a binocular,” which the name itself suggests.
In fact, the main differences between the two can be boiled down to size. A monocular will always be smaller, lighter and easier to carry than a comparable binocular. Less than half of that, in fact, since the ridge holding the two barrels together won’t be present. A single cylinder will also be a lot easier to fit in a pocket, or in a bag with the rest of your belongings, compared to a relatively wide frame.
So if storage space is a major issue, then the monocular will be the favored option, especially as they can get pretty small in length as well. There are some excellent monoculars for sale that are about the size of a human thumb, and no wider than a hefty pen. These are great for casually carrying them around inside your pocket or purse, to be popped out whenever something requires a closer look, like the firm on a far away store or the license plates on a car. In other words, smaller monoculars can act as objects to be worn on a regular basis, instead of situationally.
Now, they *can* offer similar functionality to binoculars for outdoor use, since many of them come in the appropriate aperture size. For example, a 10×50 monocular will potentially make the image appear as close, as wide and of a similar quality to what an equivalent binocular might provide, with the obvious difference of only allowing you to follow it with one eye.
This can offer some benefits in certain conditions, but is considered a downside. First, looking through the monocular for long will prove to be much more of a strain compared to a double piece. Our eyes aren’t designed to work independently from one another, and concentrating on the image coming from either one will make you fatigue very fast.
All else being equal, the number of vision tubes used on a magnifying device shouldn’t seriously impact the maximum field of view allowed (which is how much of the background fits in the image), since this is factored by objective size, magnification, and ultimately lens shape. So on paper, it should be as easy to follow moving objects with a monocular as with a binocular.
But confronted with reality, this will usually not be the case, since both the formerly mentioned eye fatigue and your attention being constantly drawn to what the naked eye is seeing will make it hard to concentrate and keep track of what the monocular is trained on.
This can be somewhat circumvented with proper training, and it might prove less troublesome for army sharpshooters or hunters that are already familiar with using a scope, but for your regular Tom, Dick, and Harry, the binocular will always be a better option when scouring the horizon for any amount of time. That’s why mono eyepieces aren’t recommended for activities like stargazing, bird watching or hunting, but will surely prove useful for getting your bearings during a nature outing!
Another thing that might worth bearing in mind when choosing between a binocular and monocular is that proportional to their size and weight, the single barrel variety will always be more powerful, assuming all else to be equal. This might tip the scales if you want maximum magnifying power and an acceptable field of view from a unit you aren’t expecting to contemplate the scenery with. If the eyepiece is only used for taking a snap at a far away peak, for example, then there’s no reason to have a second tube to take up space.
Monoculars intended for outdoor use have the same functionality as their doubled cousins. They have relatively large objectives, commonly sized between 35 and 50 mm, to allow for a lot of light in, coupled with a high magnification power (8x, 10x, etc.) and special reflective coating on their lens and prisms to give them the best brightness possible.
Sturdy frames are used, generally coated in rubber to resist accidental impacts with rocks or tree branches, and a level of fog and water protection is added, so that the inside of the lens doesn’t condense. Some of them are even safe to be submerged and can float.
However, smaller units are usually found to be preferable for the convenience they offer, and maximum magnification tends to be traded for ease of handling and eye comfort. Since it’s considered particularly hard to keep a powerful monocular trained on a target without a tripod, a magnification of 6x is found to be ideal by most users to give you a steady image that’s easy to follow.
These smaller units can also effectively double as magnifying glasses to a far greater extent than binoculars. First, monoculars allow for a smaller minimum focus, in the range of inches instead of feet. Second, most models can be simply turned around, and the near objects on the other side will appear clear.
A final thing that might recommend a monocular is your own vision’s peculiarities. This might seem obvious, but people that suffer from poor or no vision in one eye might not need two oculars.