Canon 10 x 30 IS Technology Guide - Page 11

Exit Pupil/Available Aperture of Objective Lens/Twilight Coefficient 3) Twilight Coefficient Watching a bird from a distance at night, you would use a 300 mm f/2.8 instead of a 50 mm f/1.4 lens, even if the brightness were cut in half. The amount of gradation which the human eyes can see is very large, and with experience, will be able to distinguish between the colors. In this way, although the eyes may be weaker when bright, the higher the magnification (resolving power), the more detail the eyes will be able to distinguish. A simple calculation rule: Twilight Coefficient = Magnification x Available Aperture of Objective Lens (the brightness is limited to a certain point) For example, if you compare the brightest binoculars for general use, those of 7x50 (exit pupil: 7.1 mm) and 12x36 (exit pupil: 3.0 mm), the binoculars of 7x50 are brighter. But with the twilight coefficient: 7x50=18.7 12x36=20.8 As a result, using 12x36 binoculars at night will provide more detail. Brightness and Lens Reflection Even with binoculars with the same exit pupil, the brightness may not always be the same. This is due to the amount of light reflected by the lens. With uncoated lenses, about 8% of incident light is reflected. The more light is reflected, the lower the brightness of the images will be, in addition to a decrease in image quality.In order to prevent the incident light from being reflected, Canon uses a "super spectra" coating, which has been highly praised for its efficacy with Canon's EF lenses. Canon also coats the prisms as well to ensure Left: lens not coated. Right: lens coated. The lens bright and clear images. on the left looks white because light is reflected. 11