A selection of high quality compact binoculars ideal for bird-watching and other terrestrial application. The 8x42 and 10x42 sizes are considered optimum here. The first number (i.e. 8x or 10x) refers to the magnification, the second (i.e. 42) is the optical diameter or aperture in millimeters. Larger lenses give better image brightness, especially in low light (dawn or dusk) applications, and also give better image resolution, but are of course heavier. Roof prism binoculars (the straight-through type) are generally lighter but cost more to manufacture. Porro prism binoculars fit nicely into the hands but not necessarily a pocket, but tend to cost less.