All Binoculars are created to meet certain specifications. These are indicated by numbers.... for example ....15 x 70 or 8 x 30 or 25 x 70.....
But what do the numbers mean?
The Binocular numbers indicate their strength of magnification and size of opening.
This means... the first number gives the binocular magnification strength. A binocular with a magnification strength of 15 indicates that the object you are looking at will be magnified 15 times. So, if you are looking at an eagle, it will look 15 times closer to you than if you are looking at it with your bare eyes. Now... while it is very nice to have things magnified... keep in mind, the higher the magnification, the more sensitive the image is to movement. (If you are kind of shaky, it will translate into a 15 times more shaky object.)
The second number is the width measured in millimeters across the objective lens (that's the lens closer to the object you are looking at....).So, the 15 x 70's have an objective lens 70 mm across. The bigger the number, the more light is let in.The more light... the clearer the object. When you are looking at stars or other deep sky objects (like the moon), you need as much light as possible. A bigger objective lens is generally better.
And now there is one more number to consider... the Exit Pupil (no, this isn't a kid sneaking out of class). The exit pupil is a calculation done by dividing the measurement of the objective lens by strength of magnification. (Don't worry, it's pretty easy as math goes.... using the 15 x 70 example, you have an Exit Pupil of 4.6666...yes, I'll let you round up to 5 ).
Now.. how is this important or useful?
The Exit Pupil determines how much light leaves the binoculars and goes in to your eye. (I realize this is getting technical, bear with me a bit longer, it will all come together....And I promise there will not be a quiz). So, think about the human eye... our pupils dilate and expand according to the amount of light. The average pupil width is 4 mm. If it's a bright and sunny day, our pupils will shrink down to 2 mm in diameter. If the light is low, our pupils will widen up to 7 mm in diameter. The width of light sent by the exit pupil should roughly correspond to the activity (and pupil width) for optimal viewing.
This means... if you are outside on a sunny day your pupils will shrink, and an exit pupil under 4 mm is great. If light is low (like it generally is when you are stargazing) you want a larger amount of light sent to your pupil... so you want a bigger exit pupil. Over 4mm is good.
One more thing... if you are holding binoculars in your hands, the binoculars are subject to the movement of your hand. Even when you are holding REALLY STILL, you are moving. Just a teeny bit. A Exit Pupil number over 7 mm will keep the image from bouncing around in your eye too much (bigger target is easier to hit).
Hope that helped!