It´s the baffling, stupid
It could well be that internal baffling is the main factor determining binocular brightness, surpassing and sometimes contradicting any increases in transmission by new glass or coating technologies. Baffling will result in artificial vignetting and strongly baffled bins will have a darker image by blocking some not on axis light rays. This can be nicely seen in out of focus highlights that turn from bright circular patches on axis to cat eye shaped patches towards the periphery of the field.
On the plus side, contrast will increase as there is less flaring and ghosting, and perceived contrast will also increase by crushing the blacks, that is making dark parts of the scene appear even darker in the image.
This might be the key to understanding Nikon´s strategy in their EDG series, but also in the SE 8x32. I was puzzled to find the EDG 8x42 to be phenomenally contrasty but visibly darker than its competitors, and this was not consistent with the good transmission figures of the EDGs. But then, transmission measurements should not be affected by overaggressive baffling, as the measurements are made on the optical axis. That is why I now think that transmission curves are pretty useless to determine the brightness of a binocular´s image, although they are very interesting with regards to color reproduction.
The same thing as in the Nikons can be found comparing the Leica Ultravid HD Plus 8x42 with its 8x32 sibling. The latter is overbaffled with truncated exit pupils, so images are extremely crisp, but visibly darker even in daylight with observer´s pupils closed to 2.5mm.
The phenomenal contrast and flare suppression of the EDG will be especially great in very bright environments (say, African safaris). For darker regions like the one I live in, the Ultravid 8x42 seems brighter and nicer to my eyes, although (and also because) it flares more.
Swarovski binoculars generally have very weak baffling, resulting in more flare, which brightens up the image. The big advantage of this strategy seems to be that ease of view is as good as it gets, as the exit pupil can easily be found under most circumstances.
Zeiss and Leica tend to be somewhere inbetween the Nikon and Swarovski extremes. The Zeiss HT is a phenomenal glass and the brightest of all 8x42s, but then, the Ultravid has better ease of view and is much more compact.
The Nikon EDG is really a fine example illustrating a simple truth regarding premium binoculars: All manufacturers put a lot of effort in their top bins, but they have different priorities, and the product of these special qualities becomes the soul of the bin.The EDG 8x42 puzzled me with its reduced brightness, and I cursed Nikon for its coating technology which seemed outdated compared to other brands. Which is nonsense. Nikon wanted the EDGs to be extremely contrasty, even if that meant a slight loss in image brightness by baffling.
It will be very interesting to see what Leica did in the Noctivid compared to the Ultravid and EDG...