So you’ve decided to get a set of binoculars and you want to know the ins and outs?
Binoculars come in a number of shapes and sizes. One can find a pair that will fit in a shirt pocket or a pair that requires a trailer to move them. My guess is that you are looking for something in between. Let me start by saying that any pair, no matter how small or inexpensive, will show you more then you can see with the unaided eye. With that said let’s get into the meat of the matter.
As I have mentioned previously the things you want to look for in choosing binoculars are magnification, aperture, and coatings. The first two items are easy to find when shopping. These will be listed on the information about the binoculars in the form of magnification X aperture (i.e. 7X35). If you are looking at a pair of 7×35 binoculars it means that they have a magnification of 7 times and aperture of 35 millimeters conversely, 10X50 means 10X and 50mm and so on. This brings us to another point. The fully dark adapted healthy eye opens up to 7mm. What this means to you is that you want a pair of binoculars that gives you as close to 7mm of light as possible. If you have more then this you are wasting aperture and any less you are not using your whole eye. Most binoculars give off 5mm of light to your eye. This is generally acceptable for every day use. You can determine the amount of light given off or exit pupil, by dividing the aperture by the magnification. For example, 35mm/7=5mm, 50mm/10=5. Anything less then 5mm will be unsuitable for astronomical use. You can now find binoculars with a configuration of 9X63 and 10×70 which will give you an exit pupil of 7mm. Another thing to consider, is that as aperture increases the weight of the binoculars increases. This can make it more difficult to hold the binoculars steady and can tire your arms out quickly.
The other key thing to consider when choosing binoculars is the type of coatings they have. Glass, as it is, reflects light. To prove this point just take a look at your windshield on a sunny day. By adding coatings to the glass, the amount of light reflected can be decreased. All binoculars have some type of coatings. As a rule of thumb, the better the coatings, the more expensive the binoculars. Most of the lower end binoculars, less then $50, will have a single coating on the outside lenses. Over time these coatings will wear off and limit what you can see. As you move up the price range the binoculars will have multicoatings then become fully coated and finally fully multicoated. A fully multicoated pair of binoculars will allow from 95-99% of the light to pass through to your eye and can be in the range of more then $250. Coated optics give off a green or reddish hue when looked at from a distance.
The last thing I want to mention about binoculars has to do with their construction. In order for binoculars to give a stereo upright view they have to bend the light. This is done through the use of prisms. Most binoculars use porro prisms to accomplish this. The porro prisms are what give the binoculars that fat look. Another type of prism is the roof prism which allows the binoculars to be more streamlined. Whether the binoculars have roof or porro prisms is really unimportant however, the type of glass used to make the prism that is used can affect your view. There are two basic types of glass used today the Bak4 and the Bk7. The Bk7 is the standard glass found on the market today and can cause the exit pupil to have a gray or square edge to it. Bak4 glass is more refractive and can therefore allow more light to be focused on the pupil giving it a crisp round shape and better visibility.
All these things considered, my recommendations are to get the most out of what you can afford. If you are on a tight budget find a smaller pair of binoculars with better coatings rather then a large pair with poor coatings. For the beginner or budget minded person a fully coated pair of 7×35’s found at most sporting goods stores will suffice. For the intermediate amateur a pair of fully coated or fully multicoated 10×50’s also found at most sporting goods stores will do nicely. The avid amateur may want to consider a pair of fully multicoated 12×80’s or 15×100’s with a tripod mount. The most important thing to remember is to have fun and don’t become obsessed with the details.