Buying a monocular can be pretty intimidating with all the technical manufacturers give for their product, and some research is required to ensure that you get the right one for the task you have in mind. As we’ll see, simply going for the biggest numbers just won’t do in the case of optics, and selecting them based on cool-sounding features that you won’t use will just needlessly empty your wallet. To get you properly informed, we’ve explained some of the most important specs and features to look for when choosing a monocular.
Colloquially referred to as the “power” of the monocular, the two numbers that are most commonly associated with this particular instrument refer to the magnifying power and the size of the objective — which is the front lens of the piece. The magnification factor will appear with an x after it, followed by the number for the objective diameter, expressed in millimeters.
Deciding on the right magnification is the first choice you will have to make. Naturally, a higher magnifying factor will let you clearly see things further away, but it will also decrease your field of view and make the piece hard to steady. It’s quite a challenge to keep the image from shaking with 8x monoculars, and most people can only effectively use a 10x piece attached to a tripod. If you’re looking for a new monocular for hiking, then a 6x factor will usually do quite well. You can get away with even less than that if your intended purpose is casual use, around parks and urban areas.
The objective lens diameter determines brightness and is correlated with the magnification factor, as powerful monoculars need a lot of light to enter the aperture for a clear, bright image to be formed. You should look for big lenses if you intend to use the monocular at night, but be warned that this comes at the detriment of portability, since a wider objective means a more sizeable, heavier frame.
The field of view refers to the portion of the horizon you’ll be able to see through the monocular. It is either expressed in units of size, at a set distance of 1000 yards or at an angle. This further breaks into the apparent angle, which denotes how wide your vision cone will be if hypothetically sitting in a spot as close to the image as the monocular makes it appear; and actual angle, which is self-explanatory, and expectedly much smaller. Most manufacturers use units of size and sometimes the apparent angle to denote the field of view, while the actual angle is almost never specified.
Field of view is particularly important if you need to keep track of fast moving objects, such as birds, but seeing more of the background will increase your situational awareness in any circumstance.
Besides the shape of the lens and magnification factor, the field of view is further determined by the type of prism the monocular uses. There are two common designs, the Porro, which has been around for a couple of hundred years, and the slightly newer roof prism. Out of the two, the Porro naturally offers a clearer, crisper image and a wider field of view. Where it fails, however, is convenience and portability. Porros are regarded as frailer, and they make for a bulkier design as the objective and the exit lens are not in line. The roof system allows for a streamlined design, with a little protuberance on one side to fit the prism.
The glass used for the prism also plays a role in the quality of the image you’ll be getting. If you want the best clarity and brightness available, then you should go for BAK-4, that has nearly no internal reflection. Today, this glass is cheap enough to be used even on affordable models, but if you’re really stingy with your wallet than the less performing BK-7 will have to do.
Glass is naturally reflective (around 4% of the light received), which can negatively affect clarity, but also brightness since less light will end up hitting the retina of your eye. A magnesium fluoride coating is used to combat this problem, applied to either one or more of the surfaces light will be hitting. If a manufacturer specifies a monocular or binocular as “coated,” this means that not all the optics inside have received this treatment.
When all the potentially reflective surfaces have been coated, then the piece will be deemed “fully coated.” Sometimes multiple layers are applied on one or more surfaces, which will rate the monocular as “multi-coated.” If all glass bits have been multi-coated, the instrument will be deemed “fully-multicoated”.
The “exit pupil” refers to the “rod” of light that will be hitting your eye after passing through the optic device. Its diameter should be at least as large as that of your pupil at the time. Otherwise, you’ll be seeing a dark border around the image. The exit pupil is sometimes specified but can also be easily determined by dividing the diameter of the objective by the magnification. For example, an 8×42 piece will have an exit pupil of 5.25 mm. You generally won’t need more than that, since a human pupil will only grow to about 7 mm in near pitch black conditions.
Eye relief will be important for glass wearers, as it denotes the furthest point from the exit lens at which the image will appear in focus. Most regular framed glasses sit at around 14 mm from your eyes, so check that the monocular allows for at least that.