MM is right, if you divide your objective lens by your occular lens you will have what they call your "exit pupil" . 10x42 = 4.2 An exit pupil of 5 is considered nice bright glass & there is debate on how much more light your eye can benefit from. I would think that a pair of 8x40 byLeica orKahles would be ultra bright, razor sharp& get you deep into the shadows. Here's a link to some great info :

http://www.holgermerlitz.de/