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.
- Effective diameter
- Below 25mm:Compact-type binoculars
- 30 - 49mm:Standard binoculars
- Over 50mm:For astronomical observation and business use