Customers ask – Experts answer: Exit pupil diameter and light transmittance

Customer question: I'm on the lookout for a new pair of binoculars, which should be as bright and true-to-life as possible. What values do I need to consider?

ZEISS answers: Imagine you're planning to build a house. To make sure the rooms are bright, the windows should be as large as possible - right? That brings us to the first factor, the so-called "exit pupil diameter". This is calculated by the formula "lens diameter divided by magnification" and basically specifies the diameter of the light beam exiting the eyepiece. By way of example, at 10 x 32 it's 3.2 mm, and at 8 x 56 it's 7.0 mm. You can compare this geometric value to the size of a window in a house.

The window might be huge - but how clear and clean it is is described in binoculars by the transmission value, meaning the percentage of light that actually passes through it. Premium binoculars have values around 90%. The VICTORY HT binoculars (HT stands for High Transmission) set new standards with their new SCHOTT glass, T* coating and the bright Abbe-König prism system. They let up to 95% of the light pass, corresponding to clear, clean windows.

How do these factors interplay? A large exit pupil diameter only makes sense if the eye pupil is also wide open, so all the light can reach the retina. Which is mostly at twilight. In contrast, the advantage of high transmission is always plain to see. Of course the gain can be felt the most at twilight, when it's all about achieving maximum light efficiency. Yet also during daytime, when the pupils are only opened about 2 to 3 mm wide, there is more light available, ensuring that even in the shade all details can be seen clearly and in full brilliance.

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