What do the numbers mean when binoculars buying?

Louie Asked:

I am looking for a good set of binoculars buying  for hunting. Can anyone help?

Reply:

Let's say when your binoculars buying you are looking at a pair of 10x42mm binoculars. The 10x refers to the magnification. In this case, 10x. The 42mm refers to the objective diameter, in this case 42mm. If you divide the diameter by the magnification, you get the exit pupil. In daylight your pupil is 3-4mm. You want an exit pupil no less than 3mm. 4mm is much better, otherwise you will see the edges blacken. At night, order prescription drugs online your pupil opens to at least 5mm. You want an exit pupil no less than 5mm.
The larger to diameter, the more light gathered. On a clear night, look up. You see plrnty of stars. Then look up with a x50mm pair of binoculars. It is like habing 50mm pupils. You will see many more stars, too faint to be seen with the naked eye.

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