I needed a new pair of binoculars so I began my internet search for just the right pair. But then I saw those numbers...7 x 50mm, 10 x 25mm, 15 x 40mm. So what does it all mean? I looked it up and it's really quite simple. This is what I found:
The first number is magnification. That means how many times closer the object will appear. So if your binoculars are 7 x 50, the object you're viewing will appear 7 times closer. Keep in mind that the higher the first number is, the narrower the scope of vision.
The second number is the size of the objective lens diameter in millimeters. The objective lens gathers light to allow the object to appear brighter. The second number has nothing to do with magnification, just brightness. The size of the objective lens also determines the size of the binoculars. If you want a small pair of binoculars, look for a low second number.
The exit pupil number is usually shown on the back of the box, it doesn't have anything to do with the first and second numbers such as 10 x 25. The exit pupil is the size of the hole through which light travels to the eye. Simply put, the larger the exit pupil, the brighter the object.
That's it! I hope you find this useful when you're ready for a pair of binoculars.