Binocular Magnification Light Power Ratio

Binocular relative brightness index

The relative brightness index (RBI) gives you a fast read on how bright a binocular's image will look once the light goes. It is the exit pupil squared, and the exit pupil is just the objective diameter over the magnification: RBI = (D / magnification)². Run a 7×50 through that and the exit pupil lands at 50 / 7 ≈ 7.1 mm, which squares to an RBI near 7.1² ≈ 51.

Why the square? Because the light reaching your eye tracks the area of that exit pupil disk, and area grows with the diameter squared. A higher RBI puts a brighter image on the same target. Around 25 (a 5 mm exit pupil) works well by day. Push up to 49–50 (a 7 mm exit pupil) and you are in astronomy and dusk territory, although your own pupil rarely opens past 7 mm, so anything beyond that is just thrown away.

Applications

Lean on the RBI when you are weighing binoculars for dim conditions, whether that is stargazing, birding at dawn and dusk, or work on the water and in the field. Just remember what it leaves out. It says nothing about lens coatings or glass quality, and the twilight factor plus the actual coatings shape how bright the view really feels.

FAQ

Is a higher RBI always better? Only as far as your pupil can keep up. Once the exit pupil grows wider than your eye dilates (roughly 7 mm in youth, less as you age), the iris blocks the surplus light before it ever reaches the retina.

How does it relate to exit pupil? It is nothing more than the exit pupil squared. A 5 mm exit pupil lands at 25, and a 7 mm one comes out around 49.

Does RBI account for coatings? It does not. Two binoculars can share an RBI and still look noticeably different in the field, since anti-reflection coatings and the type of glass change how much light actually gets through.