Don't get too carried away with the large objective lens. The
rule of thumb I learned years ago is that the objective lens in
millimeters divided by the magnification should be greater than or equal
to 5 ( for example with 10 x 50 binoculars 50 / 10 = 5). IT IS NOT TRUE!
This number is called exit pupil and is the size of the spot of light
entering your eye (if your eye is in the right place). Those 10 x 50
binoculars produce a 5 mm image at your eye. The typical eye pupil varies
from 2 to 8 mm depending on light conditions. Most of that variation
occurs under very low light conditions (ie at night) so most of the time
your eye is in the 2 to 4 mm range and your eye is not gathering all the
light from the 5 mm image. Consequently while I would agree that the 8 x
20 binoculars Christine mentioned are going to have light gathering
problems, 8 x 30 binoculars may be quite good unless you go birding at
night. There might be a 15 minute twilight window at morning and night
when you can tell the difference. My belief is that the rule of thumb of
5 did not come from birders but from other users (military perhaps?) where
VERY low light conditions were more common.
On the other hand there is one advantage you really do get from
larger exit pupil which is that your eyes do not have to be as well
aligned with the binoculars. This is particularly important if you often
share your binoculars with someone else whose eyes are a different
distance apart.
So don't be scared to get binoculars with exit pupils as low as 4
or sometimes even 3 but DO try them first. The inside of many stores
actually is low light conditions so look at things in dark corners of the
store.

Jim Elder (jime at eskimo.com)
SEA, Inc.
7030 220th St SW
Mountlake Terrace, WA 98043