The second number in the product’s name represents the lens diameter. For example, 10×42 binoculars have a lens diameter of 42 mm. It is known that the bigger the lens, the more light goes through and the image we see is brighter. All of this, however, also depends on the magnification and quality of a certain optical product. Although a bigger lens diameter in binoculars is better, the size adds up on the weight, making it heavier and more difficult to handle.
The most common lens diameters are 24 mm, 42 mm, 50 mm, and 56 mm.