What does the second number in an 8 x 40 binocular mean?
A3. The second number in a binocular – the 40 in 8 x 40mm or 50 in a 10 x 50mm – is the diameter (width) of each front lens (commonly referred to as the ‘objective lens’ or ‘objective diameter’), measured in millimeters. This number is also referred to as Aperture, and is derived from the Latin word aperire (“to open”). The objective lens size, or Aperture, is the determining factor for the amount of light captured in an image and has a direct impact on binocular performance. When magnification and quality is equal, larger objective lenses will produce brighter and sharper images. An 8×40, then, will produce a brighter and sharper image than an 8×30, even though both magnify the image an identical eight times. The larger objective lenses in the 8 x 40 also produce wider beams of light (exit pupil) that leave the eyepieces. This makes viewing more comfortable with an 8×40 when compared to the 8 x 30. By allowing more light to be captured in an image, the larger objective lenses are idea
