The effective diameter is the inside diameter of the objective lens frame. With the binoculars designated with a numerical formula 8x42 7.0°, 42mm is the effective diameter of the objective lens.
Given the same magnification, the larger the objective diameter, the greater the light-collecting power. This results in higher resolution and a brighter image. However, large-diameter objective lenses make binoculars bigger and heavier.
Binoculars are classified according to the effective objective lens diameter as follows.
Below 25mm objective diameter are compact binoculars & monoculars used most for sport games, operas, concerts, art galleries, revealing hidden details while meandering around the museums of this world.
30 to 42mm objective diameters are used for lightweight travel, birding and standard observation. 42mm is considered the best all-around size, offering great light gathering capabilities, while also being compact and easy to travel with.
50mm to 55mm is mostly used for hunting and bird watching, offering more brightness than 42mm and better low light performance. The magnification at this range is also most of the time higher between 12x and up to 15x.
55mm and up is specifically for astronomy and sky observation