When choosing a monocular scope, consider its size and weight, the magnification, and eye relief. The size of the objective lens will also determine whether a particular model is a good choice for you. There are also specialty monocular scopes available, such as zoom monoculars, that are handy when extra magnification is needed.
The biggest advantage of a monocular scope is its size. It is compact enough to fit into a pocket or handbag. The monocular scope has just one barrel. Monocular scopes range in size and can be as small as 3 inches (about 7.5 cm) in length and as thin as a pen, so consider what you'd like to use the monocular for and where you plan to store it.
When purchasing a monocular scope, you should understand the level of magnification and how to identify scopes that offer higher or lower magnification. Monocular scopes are marked to show what the magnification is. A scope with the markings "5 x 10" offers a magnification of 5x, which means that an object will seem five times larger when using the scope.
The most practical magnification levels for monocular scopes would be 5x or 6x. The higher the magnification, the more restricted the field of view, which makes the scope more difficult to use. A rule of thumb is to choose a lower magnification if you need a wider field of view. Some monocular scopes offer magnification of up to 10x, but these are harder to steady and harder to spot the object with.
Eye relief refers to the space between your eye and the eyepiece while still able to see the entire field of view. It is measured in millimeters and is a very important feature to consider if you wear glasses. An eye relief of at least 14 mm (about 0.5 inches) is recommended for individuals who wear glasses.
The term objective size refers to the size of the objective lens, and it is easy to check the size when you look for the magnification level. The second number is the size of the objective lens. In the example above, “5 x 10,” the objective size is 10 mm (about 0.3 inches). Bigger is not always better when it comes to objective size because a bigger objective lens makes the monocular scope bigger and heavier.
A zoom monocular offers higher magnification levels and is better suited to those applications where smaller or slow moving objects are viewed. Zoom monoculars can go well beyond magnification levels of 10x, but they are much harder to use than ordinary monoculars. The field of view is very limited which makes them unsuitable for general use. When purchasing a monocular scope, it's good to keep durability in mind, and zoom monoculars tend to not be as durable.