It is my understanding that a lot (most?) of the perceived difference in brightness among alphas is due to skewing of the transmission curve in some binoculars. A boost in yellow-green color causes a perceived boost in contrast and brightness. The following is taken from greatestbinoculars.com and may be useful to you:
"The problem about colour rendering in binoculars is that the designers have a conflict here, especially when designing a glass for nature observation, not hunting. They need to decide between colour accuracy and a perceptual boost in contrast and brightness by skewing the transmission curve and introducing colour casts.
A green or yellowgreen cast increases the perception of both contrast and brightness. This "glow effect" can be really amazing, especially in low light. I saw it best in the Zeiss Victory FL 7x42 (green cast) and Swarovski SLC 8x42 (slight yellow-green cast) compared vs. the Leica 7x42 UVHD+ (yellow with a touch of red). For hunting glasses, this colour inaccuracy absolutely makes sense and they will be harder to sell without it. This "glow effect" probably in combination with high contrast of textures is what I wrongly called "high transmission sparkle" in 2015.
Zeiss has always been heavy handed with their green cast and unfortunately continue to do so in their SF line, which is advertised as being designed for nature observation, where colour accuracy should be important. I can´t force them to be more subtle, and they can´t force me to buy their stuff. Fair enough.
The SLC uses a well balanced slight yellowgreen cast to great effect, the contrast and especially the brightness boost are amazing. Well balanced because the predecessor seemingly had a stronger cast (see transmission curves on allbinos). Swarovski used to advertise the SLCs as "contrast optimized". I would avoid older Swarovski models, as most of them had a strong yellow green cast being contrast optimized for hunting.
I perceive the SLC as the brightest 8x42 with SP prisms. The disadvantage is that the image lacks in blue and red, whereas the Ultravids deliver full red. This is very noticeable in autumn landscapes. The Ultravids still have a bit of "glow effect" from their yellow cast compared to more neutral glasses (Swarovision, EDG). For my taste, this is the sweet spot of colour rendition and contrast in binoculars, and this autumn proved it again and again.
But compared to the SLC, the UVs seem to have a darker imagery, because the "glow effect" is reduced. With the SLC, I always think the image is even brighter than what I see with naked eyes, with the UV7x42HD+ or 8x32 I always feel it is a bit darker.
The Nikon EDG and Swaro SV are more neutral, neither cutting of a lot of blue nor red. This can be very beautiful, especially in brighter scenes, but in low light they lack any "glow effect". I have a rose with creamy pink flowers, it looks most gloriously natural viewed through the Nikon EDG, but it does not glow in twilight with the Nikon.
The Leica Noctivid in my view has introduced a bit more green compared to the Ultravids, giving it a stronger glow in lowlight at the expense of magical Leica reds. They could have pushed much harder, like the SLC, but colour accuracy was obviously more important. I can assure you that the SLC images look much brighter than the Noctivid in low light. The Noctivid is a very good compromise colourwise, but that can´t be fully appreciated due to other issues (flat field, diffuse flare)...
Interestingly, Swarovski tried something else with the NL Pure: Neutral colour with highest transmission. It will be interesting to see how this works out, but I bet against it. You can´t get a strong "glow effect" with neutral colour, and a small percentage of more brightness will be hardly noticeable.
I appreciate the SLC more in low light and blue light, as it is hands down the brightest glowing 8x42 available new and the very best 8x42 for hunting - the Zeiss Victory FL and HT are no longer produced. Very bad move from Swarovski."
The full article can be read
here.
