Transmittance of Binoculars
As light travels through a binocular, a certain percentage of that light is lost through absorption and reflection at each air-to-glass surface or inside the prism system itself. The amount of original light available to the observer by the time it exits the eyepiece will vary from as low as 50% to as much as 97%, depending on the quality and number of optical glass elements used in the lenses and prisms, configuration and size of the prisms, collimation of the optical system, and type and amount of anti-reflection coatings present. This is an important factor that directly effects the actual brightness of the observed image. The term used to describe this percentage of light that is not lost through the optical system is transmittance and for most quality binoculars this figure will usually be above 90%. With this factor taken into account, it's possible for a 10 X 40 binocular (exit pupil 4mm) with a high transmittance (90%) to actually deliver a brighter image than a 7 X 35 (exit pupil 5mm) with a lower transmittance (70%).
Thanks to our friends at Eagle Optics for providing this information!
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