Binoculars come in a great variety of sizes and magnifications. Compact, pocket sized models are usually 8x25 or 10x25 - the first number is the magnification and the second the diameter of the objective lens at the front. Popular binoculars for bird watching would be 8x42. The larger the second number the more light will enter the binoculars, making the image brighter and clearer. There are very high powered binoculars around which need to be mounted on a tripod for the best results and these are ideal for general observation and astronomy viewing - especially the moon.