What is a Relative Brightness Index and why is it important?
The Relative brightness index is a measurement of how bright a binocular will be given its power and objective lens size. It is calculated by dividing the size of the objective lenses by the power and finding the square of the resulting number. For example, the relative brightness index or RBI would be calculated like this (42/10)²=17.64. It is important to note this value in a binocular when brightness and clarity are important. While one might think that a 10×42 binocular is brighter than a 7×35 because it has larger objectives, the 7×35 would actually produce brighter clearer images. This fact can be seen when comparing the RBI values: 10×42⇒17.64 — 7×35⇒25.
