At night, a large objective lens is what makes binoculars amazing or inadequate. The exit pupil is determined by dividing the obj lens diameter by the power. 40/7=5.7 vs 50/7=7.1
A 7mm exit pupil vs slightly less than 6mm exit pupil. Given the same glass, 7x50's will beat 7x40's every time. For brightness always go with a big lens up front. My 15x80 Steiners only have a 5.3mm exit pupil, but that 5.3mm contains a lot of light.