Does anyone know if ED glass affects either of these parameters? perhaps important if you are looking at travel scopes?
For background I copied the info below
RELATIVE BRIGHTNESS INDEX (RBI)
Relative brightness index or RBI is used as a comparison of image brightness but it does have severe limitations as discussed below. It is determined by square root of the exit pupil. An RBI of 25 or more is considered useful in low light conditions. In bright light conditions an RBI of 9 would have no brightness advantage over one of 50.
RBI = Diameter / Power = exit pupil x square root.
Example : 7x50 binocular 50/7 = 7.14 x 7.14 = 50.98 = relative brightness.
Twilight factor is a measurement of viewing sharpness and image detail in low light conditions. The larger the twilight factor, the more efficient the low light performance. This indication has some limitations. To calculate the twilight factor, take the square root of the power multiplied by the objective lens diameter.
all input welcome mike