Hi Hampus. I'll add more comments and recommend that you read the Wikipedia page on exit pupil. One commenter mentioned that a higher power binocular has the image "spread out" more and thus is dimmer. True to an extent, but not the whole answer. It depends on you individually and especially on your age. Realize the following:
Exit Pupil = objective lens diameter divided by magnification. Thus a 12X50 bino has a 4.17mm exit pupil. 10X50 is 5mm. And 8X50 is 6.25mm. However, according to the Wiki page, a 20-year-old person has an average eye pupil diameter of 4.7mm in the daytime and an 80-year-old person has an average eye pupil size of 2.3mm. Huge difference. No wonder that some old folks don't like driving at night! By my loose understanding of this, if you are 80, much of that 6.25mm exit pupil from an 8X50 would fall outside the edge of your eye pupil. Perhaps someone well-versed in physics/optics can objectively clarify this, but it seems that age is a big factor. Perhaps much of the brightness of an 8X50 (compared to 10X50 or 12X50) would be lost on me now at age 75. That Wiki page has a table showing that even at age 40 the pupil diameter is lower than at age 20 (3.9 vs 4.7mm) and both smaller than that 6.25mm exit pupil of an 8X50 bino. Bottom line. Try before you buy. And if you're already 50 or 60, keep in mind that your eyes will change as you get older. Frankly, I don't see much brightness difference at dusk between our 12X42 and 10X42 binos. I have one other reason for the try-before-buy advice. Many people rave about some binos having great edge sharpness. When I compare my 12X42 NL to our 10X42 Zeiss Conquest (besides the $3000 vs $1000 difference), yes, the NLs are razor sharp clear to the edge. But I rarely spend time looking at the very edge of the field of view. I simply turn my head a little. The center part of the human retina (fovea) with the highest visual acuity is very small. (Wiki has a page on that too.) The center of the fovea with the best visual acuity is only 0.35mm diameter. For a dramatic demonstration of this, focus on one letter in the middle of a word and then see how many words you can read on either side of that word without moving your gaze. Even one inch on either side is a challenge. (and NO cheating; keep your gaze steady) If you're some mutant who can, then try focusing on a word and seeing how far away you can recognize words without even slightly moving your gaze. Looking at binos
in person will tell you how important edge sharpness is. Best wishes.