When choosing a good pair of binoculars, consider the lens size, lens diameter, magnification power, exit pupil and relative brightness. Also consider eye relief, field of view and weather-resistance features. Also, choose binoculars depending on your specific use.
Binoculars are available in three objective lens size: full size (10 x 50 and 8 x 42), mid size (10 x 32 and 7 x 35) and compact (10 x 25 and 8 x 25). The first number indicates the magnification power and the second the lens diameter.
Full-size binoculars are suitable for low-light, wide-field viewing such as during bird and wildlife-watching and on boats. However, they are heavy and big in size. Compact binoculars are lightweight and suitable for daytime activities but become uncomfortable when used for long.
Mid-size binoculars are best suited for sports use and wildlife-watching. They have a lens diameter that can capture sufficient light to afford a bright view even when light is poor. Binoculars used outdoors with hands should have magnification power between 6 and 10. Higher magnifications cause unstable images and narrow the viewing field.
Select binoculars with high exit pupil and relative brightness for low-light viewing, such as when watching the night sky. Choose binoculars with eye relief of at least 10 millimeters or longer if you are wearing glasses. Opt for binoculars with water and fog-proof properties for unhindered viewing in different weather conditions and with multi-coated lenses to decrease reflection, increase transmission of light and to make images sharp.