Binoculars are categorised by their magnification and their objective diameter.
e.g 7x50. In this instance the magnification is 7x, and the diameter of the front objective lens is 50mm.
MAGNIFICATION: The amount of magnification should depend on the use the binoculars are designed for, hand held Binoculars have lower levels of magnification so that they will be less susceptible to movement, a larger magnification leads to a smaller field of view.
OBJECTIVE DIAMETER: The diameter of the objective lens determines how much light can be gathered to form an image, this number directly affects performance. The larger the objective diameter is, the brighter and sharper the image will appear. Comparitively, Binoculars with larger objective diameters also produce wider beams of light at the exit pupil, which makes the image more comfortable to view.