Dries 1, post 45, Not necessarily and in the best case they are identical or almost identical. I have mentioned that before: when we started our transmission measurements, the companies wanted to know how we did it so we received a number of test samples to measure and they were also measured by the company itself and next to it by a company in Germany that is responsible for proper standards in this matter. We turned out to be perfect in line with the measurements of the company and with the data of the institute for optical standards. Not so bad, so we concluded that our work was OK
We just finished measurements of some historic binoculars and then you can find transmission values of 25- 40%..
With regard to the Leica BN's: it depends quite a bit in what year they were made and which coatings are used. Gary Hawkins mentions in his short study on the history of Leitz/Leica roofs, that the BN series was supplied with another/updated coating as the BA series.
With older roofs with an aluminum mirror coating or a silver mirror coating on the Schmidt-Pechan roof surface that does not have perfect light reflection: oxidation of the metal mirror can lead to decreasing light transmission.
Mike F, post 36: a transmission difference of 3% or more is noticeable as a general rule but the eye quality of the observer can also be of influence. But do not expect a sort of night view if a particular binocular has a 5% lower transmission: the image is simply a tiny bit less bright.
The spectral composition of the transmitted light also plays a role in the observed image quality: more blue looks brighter, more red slightly dimmer, so binocular makers can play with it.
Gijs van Ginkel