A monocular is a miniature, low powered telescope or spotting scope which you hold in your hand like a binocular but use with one eye like a telescope. A monocular shares characteristics with both binoculars and spotting scopes, but is much smaller than either.

Size, then, is really what a monocular is all about. Some monoculars are no larger than your thumb or thicker than a good ink pen and any monocular will slip easily into a pocket or a purse. When a spotting scope or a binocular, even a compact, is too large to take along, there is always a monocular. There is no excuse for leaving home without your monocular.

When should you consider buying a monocular? Anytime you need to keep things small and just need to take a peek, think monocular. Is that a rock or a deer out in that field? Pull a monocular out of your pocket and take a look. What does that sign across the shopping center say? Pull a monocular out of your purse or pocket and take a look. A monocular can also do double duty as a magnifier. Some actually have a very close focus, but turn any magnifier over, hold it over an object and you have a magnifier.

When should you choose a binocular, instead of a monocular? If you need to take a long look or scan an area, go with a binocular. Using two eyes, as with a binocular, produces less eyestrain than using one eye and using two eyes also improves your visual acuity. For this reason, a monocular is not a substitute for a binocular when the action turns serious. For birding, hunting, surveillance, astronomy and other involved applications, a binocular is always a better performer. The whole idea of a monocular, though, is to have something at hand when you would not ordinarily carry a binocular. I've seen and recorded a great number of birds simply because I happened to have my monocular in my purse.

When should you choose a spotting scope, instead of a monocular? Anytime you need more magnification than you can get with a binocular or monocular, go with a spotting scope. A spotting scope may, in principle, seem to be nothing more than a very large monocular, but in truth, a spotting scope and a monocular are very different beasts. A spotting scope is large and heavy and, because of its very high magnification, it must be mounted on a tripod for steadiness. That's about as far from a monocular as you can get.

Here are some features to consider in a monocular:

Magnification (first monocular number, the 6 in 6x15)

Magnification is how many times the image is enlarged over normal. More might seem better, but with any optical instrument, the higher the magnification, the harder the instrument is to use. This is especially true in a small, lightweight instrument like a monocular. A magnification of 5x or 6x is a very practical magnification for a monocular. It will be easy to steady and have a wide field of view to help in locate the target. An 8x will be significantly harder to steady and have a narrower field of view. A 10x will be harder to use, yet.

You will see zoom monoculars with magnifications that go well over 10x, but these are impractical to handhold and their very narrow field of view make them extremely difficult to use. Lastly, a zoom monocular is never as durable as a fixed power model.

Objective size (second number, the 15 in 6x15)

The second number is the size of the front lens in millimeters. The larger this second number, the better the optical performance, all else being equal. Once again, though, bigger is not necessarily better. The bigger the lens, the bigger and heavier the monocular. A 10x40 monocular will be roughly about half of a full size binocular. A 5x15 will be no larger than your thumb.

In terms of performance, optical quality is actually more important than objective lens size. In fact, you can have an optically excellent monocular that is also very small, if you are willing to pay the price.

Field of view:

Field of view is the amount of territory you see when you look through the monocular. Imagine a fence 1000yards away. If a monocular has a field of view of 372 feet at 1000 yards, you will see 372 feet of the fence. If the field of view is 250 feet at 1000 yards, you will 250 feet of fence. Keep in mind a basic law of optics. As magnification goes up filed of view goes down. If you want a wide field of view, keep the magnification down.

Eye relief:

Eye relief is the distance your eye can be from the eyepiece and still see the entire field of view. It is measured in millimeters. If you wear glasses when you observe, this is a critical feature, because you glasses won't allow you to get your eye close to the eyepiece. If you wear glasses, look for a model that lists 14mm of eye relief or more.

Close focus:

This is the minimum distance at which a monocular will focus on an object. The close focus on some models is actually measured in inches, instead of yards, making these close focus models very versatile. There is nothing quite as spectacular as viewing a butterfly perched a few feet away through a good monocular.

Lens coatings:

Lens coatings affect image brightness. The best monoculars are labeled as fully multi-coated, followed by multi-coated, then fully coated.

Waterproof:

You'll obviously want a waterproof model if you are in a boat, around water or in a wet climate, but no real need to go waterproof for normal use.