Buying binoculars is often a compromise between optical quality, price, weight, magnification and size. Binoculars are usually referred to by reference to two numbers such as 10 x 8 or 20 x 50 or 12 x 25.  The first number describes the magnification of the binoculars or how many times closer the binocular image seems compared to the non 'binocular' view. The second number describes the width of the lens in the binoculars in mm. The wider the binoculars lens or the better the quality of the lens the more light is allowed in to produce brighter and clearer images in the binoculars. Small pocket sized binoculars usually use 8 x 21 or 10 x 25 lenses to keep the size and weight down. This can also make pocket binoculars relatively expensive as good quality lenses are required to maintain image quality. More powerful binoculars are necessarily larger and heavier due to the need for a large lens size and consequential increased bulk which also allows in more light and produces a brighter image in the binoculars.