Lower-priced binoculars use magnesium fluoride, which is quite good. Better are combinations of two or three, and sometimes seven or more, different compounds, which come very close to achieving the magical thickness. There are many moderately priced binoculars that feature the enhanced performance of multicoated lenses. The best performance, however, comes with the increasingly sophisticated lens coating processes found, you guessed it, in the increasingly expensive models. They may not look much different, but you can see the difference when you put them to your eyes.
Relative brightness is important.
In order to quantify in some way the brightness of their binoculars, some manufacturers use exotic-sounding formulas to sell their products. In order to make the differences between models seem more significant, some companies use relative brightness, which is simply the square of the exit pupil. A 7x35, then, with a 5 mm exit pupil, has a relative brightness of 25, whereas a 7x42, with an exit pupil of 6 mm, has a relative brightness of 36. All you really need to know is that relative brightness is relatively meaningless. Another measure that is sometimes used is twilight factor.