Binoculars come with two types of focusing mechanisms. Most people opt for the center-focus model, which uses a centrally mounted wheel to adjust both eyepieces at once. There is also a separate adjustment for the right eyepiece, which helps to correct for any difference in near or farsightedness between your eyes.
Because focusing requires only one quick adjustment, this model is the most popular, and best suited if you want to use your binoculars for other purposes except astronomy.
The second focusing system uses individually focused eyepieces and has no centrally located focusing mechanism. Even though focusing is slower compared to the previous model, binoculars that use individual focus tend to be more rugged and less prone to moisture infiltrations.
These models are the the best choice if you plan to use your binoculars for night sky observing, because celestial objects don’t change their distance and frequent refocusing is not required.
Because cleaning binoculars can be very expensive, you might consider buying waterproof models. They are designed to resist repeated changes in temperature and humidity, making it impossible for condensation to damage internal parts of the instrument. While waterproofing might not seem very useful for astronomy, it is a must if you plan to use your binoculars for boating, hunting, hiking, or other outdoor uses.
Baffling within the binocular has the role of shielding against stray light and internal reflections, and can dramatically improve image contrast. To check if your binocular has quality internal baffling, point it at a bright surface and examine the filed of view. It should be surrounded by a black background, without additional light or shiny reflections.
Because binoculars are basically two small telescopes mounted side by side, an error in collimation (optical and mechanical alignment) can lead to numerous problems including eyestrain and double-images.
Most cheap binoculars are shipped out from the factory with collimation problems, and even quality models come out of alignment as they age and get bumped around. The only solution to this problem is to buy binoculars with quality mechanics, which will last longer before collimation errors become a nuisance.
Stabilizing the View
For any binoculars to give their best, they need to be held steady. This eliminates the constant jiggling associated with hand-holding, and allows you to see small details and objects fainter than you might think possible.
The traditional and least expensive mount for binoculars is a simple camera tripod. Most binoculars come with a threaded mounting hole and an L-shaped adapter that screws into this hole and onto the tripod. If you don’t receive the adapter when you buy your binoculars you can purchase it separately, or even make your own.
Unfortunately, this type of mounting is difficult to use and tiring for the neck, especially when observing objects near the zenith. The solution is to purchase a commercial mount especially designed for astronomical use, or go for image-stabilized binoculars if your budget allows it.
Image-stabilized binoculars have an active built in optical system that compensates for the movement associated with hand-holding. All you need to do is press a button on the top of the binocular housing, and you will get tripod-like stability with the convenience of hand-holding. The only draw-backs of these models are their small apertures, and prices several times greater than their nonstabilized equivalents.