Photograph by Chuck Baker. A pair of 7 x 50s, for instance, gets a rating of 350. This means 7 x 50s actually perform a little less well for astronomy than smaller 10 x 40s, which get a visibility rating of 400. Such a claim flies in the face of decades of conventional wisdom. The visibility factor for different apertures and magnifications.
Dots mark common binocular models.
The arrows at lower right indicate the directions in which various properties increase most rapidly. Adapted from Roy L. Bishop's article in the 1994 Observer's Handbook of the Royal Astronomical Society of Canada. Nobody questions that high power helps on objects that demand fine resolution, such as close double stars, Jupiter's moons, lunar details, and resolving stars in open clusters. But as Bishop points out, it also helps on traditionally low-power objects such as very dim galaxies and faint, unresolvable clusters. The reason is sky brightness, especially in light-polluted areas.