If money is no object then get a Swarovski. Brightest and clearest scopes I have ever looked through even though I can't afford one for myself.

BTW, scopes do not "gather" light they transmit light. Every single lens the light passes through reflects a certain percentage of that light back to where it came from and as the light passes through each lenase the light transmitted becomes less and less. This is where the highest quality glass that has been perfectly polished comes into play. Same thing with glass coatings. Make sure that you gat a scope with every single lens multi coated. These coatings reduce the percentage of reflection from each of the lenses.

After that you need to do some math. An adults eye pupil only gets so big and small. In bright day light conditions the pupil is only about 2mm in diameter. At night the pupil opens up to about 6 or 7mm. So you gotta figure out the diameter of the exit pupil of the scope. Take the objective diameter and divide it by the power. For example a 3-9X50 scope will have an exit pupil of 16.6mm at 3X magnification. With the exit pupil larger than your eyes pupil your eye is absorbing all of the light that it can. When you craik it up to 9 power the exit pupil size shrinks to only 9mm. Your eye is still able to absorbe all of the light.

Now if you had a 3-12X40 the exit pupil would only be 3.3mm at 12 power. Say it is right at dark and your eyes are fully dialated to 6mm absorbing all available light. If the exit pupil of the scope is only 3.3mm then the full diameter of your eyes pupil is not being used (almost half) and the scope will not be as bright as a scope with a larger exit pupil size.