Examine any binoculars and you’ll discover that one of several combinations of numbers separated by a 'x' will be visible — for example, 7x40, 10x50, 8x40 and so on. These numbers, which are often located close to the eyepieces, tell you the magnification and aperture of the optical system. For example a 10x50 binocular has a magnification, or 'power' of 10x with front lenses with a diameter of 50mm. It’s these numbers that reveal which objects you can observe by telling you how much you can magnify them by and how much light your binoculars can collect.
The larger the aperture, the easier it will be to see fainter night sky targets such as galaxies and nebulae. As a rule of thumb, and under decent observing conditions, 7x35 binoculars will reveal a hundred thousand stars and 10x50s will show the finer details of the Moon, planets, Andromeda Galaxy (Messier 31) and the Orion Nebula. Larger, heavier binoculars such as 20x80s and 20x100s, will open up a wealth of objects such as the fainter spiral galaxies Messier 65 and NGC 3628 and a selection of globular clusters and star-forming regions.
Some models of binocular will have their field of view printed on the barrels or somewhere close to the magnification and aperture. Given in degrees, this number tells you how wide you can see when peering through the eyepieces. The larger the number, the wider the scene. Ten degrees will allow the Big Dipper’s bowl to fit comfortably in your field of view.