Relative brightness measures how bright an image appears through the binoculars. There's a complicated formula used to determine the ratio, but all you need to remember is that the higher the number, the brighter the optics. For example, binoculars with a relative brightness rating of 37.5 allow more light to pass than a set with a rating of 10.3. The human eye can not utilize light passed by binoculars with relative brightness in excess of 49.

One thing to be aware of: the larger the diameter of the objective lens, the more light it will let into the binoculars.

At mid-day this doesn't mean much. But in early morning and late afternoon, when light conditions are the poorest, you want and need the brightest optics available.

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