I guess optics such as coatings and such must come into play as well at least with my eyes because there is a significant difference in bright daylight even between my 10x42 vs 8x42 roof prism binoculars. The 8x42 is quite a bit brighter. My family sees the same difference I do too and always asks me why the 8x42 is so much brighter!
Coatings make a big difference. In bright sunshine, my Swarovski EL 8x32 was brighter than a Leica BA 10x50. Both were in excellent condition with clear optics. But the EL transmission has been measured at 90%, the BA is no more than 77% (measurement of BN, successor to BA). The 20% relative difference was easily visible.
Edited by Bkoh, 19 September 2023 - 11:30 PM.