I enjoy astronomy and though the primary tool of choice for sky gazing is the telescope, many astronomers use binoculars for viewing nearer objects such as the moon where less magnification is called for and a wide field of view is preferable. Binoculars tend to have a wider field of view than high-powered telescopes. It's a tradeoff of sorts.
Binoculars are essentially two telescopes in tandem. They work on the same or similar optical principles as telescopes.
The larger lenses on the binoculars are called the objective lens. The larger these lens the greater ability they have to gather light. Many binoculars have 35mm objective lens while slightly more expensive models will have 50mm (or even larger) objective lens. The 50mm lens will have more light gathering ability than the 35mm lens and so will work better in low light situations. The smaller lenses where you actually place your eyes are the magnifying lens.
When you are looking at binoculars you will see two numbers such as 7 x 50. The first number is the magnification number and the second number is the diameter, in mm, of the objective lens. A larger first number means greater magnification and a larger second number means greater light gathering ability. For looking at the moon, something along the lines of 20 x 60 would be very nice but even 7 x 35 will produce images that will be very pleasing to the eye.
Not all lens are created equal so one manufacturers 10 x 50 binoculars may be inferior to the same by another manufacturer if the former used lousy glass. You can use sources such as Amazon to see what ratings actual users have given a model in which you are interested.
Here is an image of what typically is going on inside of a pair of binoculars.
The image enters through the objective lens and is routed by prisms to the eyepiece lens. The eyepiece lens magnifies the image provided by the objective lens. A focusing mechanism is provided, general in the middle of the binoculars.
Many binoculars have adjustable eyepieces on or both eyepieces lens to adjust for the users vision. They are usually marked Diopter. If the left lens is marked as such than close your right eye and focus as best you can on a distant object and then turn the adjustable ring on the eyepiece until the image is as clear as you can manage.
I hope this introduction to how binoculars work provides a good basic idea of what is going on inside of these optical instruments and makes you a more informed buyer when shopping for a pair.
Those of you with a great deal more experience than I have will hopefully leave comments to help folks further understand the subject. There are, for examples,other systems of prisms.
I have always wondered what the numbers meant like 7 x 35 but have always gotten odd answers that I knew couldn't be right. Someone at Walmart gave me a really bizarre explanation. Thanks for the article.