Divide the magnification-the power setting-into the objective lens size-the lens at the farther end away from the shooter-to get the exit pupil-the amount of light that is being transmitted to the eye. Compare the scopes you are considering against each other: The exit pupil specs. Scopes with larger exit pupil #'s will be better in lower light.
That is, a scope w/an exit pupil of 5mm is better in low light than a scope w/an exit pupil of 4mm. However, scopes w/an exit pupil of 4mm are generally still good for lighting situations encountered toward the end of legal deer hunting times.