An international standard full size student microscope provides as much as 400x magnifying power, yet such a microscope consists of a tube less then 20 centimeter in length. Sufficient light is reflected from its' plain-o-convex mirror less than 7 centimeters in diameter. In order to obtain identical brightness and magnifying power in a telescope, focal ratio of f/2.5 is recommended for an exit pupil close to 7 millimeters. Such telescope will require an aperture of 320 centimeters (3.2 meters) and a focal length of 800 centimeters (8 meters), calculating roughly with a 20 millimeters ocular. This is an increase of almost 50x in size. This shows that brightness is not limited to large aperture, nor magnification limited to long focal length. However, the 'bigger is better' formula is a design limitation that surface only in distant magnification. Focusing of distant images is more challenging than focusing of close-up images. We can prove this with a single magnifying lens that is held close to the eye. Objects further then 2/3 the focal length of the lens will be out of focus.

Company Resource:  Waterproof Binoculars, Gift Binoculars
Read more: http://www.nbbinoculars.com/news/