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Relative Brightness | Optics Trade Debates

Welcome to Optics Trade debates. In each episode, we talk about a different topic and try to answer the most common questions we receive about it. Today, we are going to talk about relative brightness.

Relative brightness is found in tables with binocular specifications. It changes with the change of the size of the exit pupil.

Most other devices have a variable magnification, so the size of the exit pupil changes with the magnification, and the use of relative brightness isn’t practical.

Relative brightness is defined as the square of the exit pupil. Per instance, an 8×56 binocular has a 7mm diameter of the exit pupil and its relative brightness is 49.

Relative brightness was a really important factor in the past. It lost most of its accuracy after the second world war because most manufacturers started producing binoculars with different types of glass, coatings, prisms… All of these factors affect the actual brightness of the binoculars.

Before WW2, binoculars could be easily compared and it was obvious which models are better for use in low light. But today it’s more or less useless because the quality of optics differs so much.

Fewer and fewer manufacturers specify relative brightness for their products.

Relative brightness ranges from 1 to 49, so the minimum relative brightness is 1 and the maximum is 49.

Relative brightness was really useful in the past because binoculars came in really strange configurations and it wasn’t obvious which binoculars were brighter.

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Explanation of the term on our website:

Relative brightness is a calculation of how bright the image should be when viewed through binoculars. It is presented as a square value of the exit pupil. 10×50 binoculars have an exit pupil value of 5.0 (dividing lens diameter with magnification). Square of 5.0 gives us a value of relative brightness which is 25.0. As the relative brightness value increases, we have a brighter image. On the opposite the lower the value, the darker the image.

In the past, relative brightness was an important value in determining the brightness of the optics. The manufacturers were using the same kind of technology and materials of the lenses, therefore, the optics were comparable. Nowadays, they use different types of lenses and modern coatings so the relative brightness has lost its meaning because the brightness of the optics depends more on the quality of the coatings than on the relative brightness.

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