Being most used to looking at stars and planets, rather than birds at dusk, I was keen to understand whether the same considerations applied to both pursuits. Here's what I found out (much of it from posts on this forum):
1. Know your own dark-adapted pupil size.
It's not obvious how to do this, but two ways are:
. Hold a ruler next to your eye in a darkened room and take a flash photo(!)
. A less severe method involving looking through a piece of card with twin sets of pinholes at different spacings, which you can find at: http://home.att.net/~jsstars/EP/ep.html (it's some way down the page).
I used the second method to find that my dilated pupil size is 6.5mm.
It's important to know your dilated pupil size as some binoculars may have exit pupils, (the size of the bright disk that you see in the eyepieces if you hold them at arm's length), larger than your pupil can let in.
2. Work out or measure the binoculars' exit pupil size.
Easy, at least in theory: Exit pupil = Objective diameter divided by magnification. So for a pair of 7x42's, the exit pupil is 42/7 = 6mm. For a pair of 7x50s, the exit pupil is 50/7=7.1mm.
In practice, some cheaper binoculars "stop down" the exit pupil size: I have a pair of cheap and cheerful 10x50s that have a 4.2mm exit pupil. This means I might as well be carrying a more compact 10x42 in terms of the light I'm getting out!
For this reason, I would recommend measuring the exit pupil of intended purchases while in the store: To do this, just take a ruler with you and measure the size of the bright disk seen in the eyepieces when holding the binoculars away.
3. There's no point in binoculars with a bigger exit pupil than your own eye's pupil size.
This is a bit contentious because very large exit pupils don't have to be aligned with your eye perfectly to let in all the light they can, but basically, if the exit pupil of the binoculars is bigger than your eye can take in, the "spare" light doesn't get into your eye.
4. The brighness you get is different for stars than for "extended objects" (like the moon, galaxies etc. in astronomy, or like birds of course). To explain:
. Stars always appear as pin points of light, no matter what the magnification of the binoculars. So, allowing for the provisos on exit pupil size above, the bigger the objective, the better, with magnification playing the part of choosing the best exit pupil size and field of view. (This is an oversimplification, but the main point is that for viewing stars, it's only the objective size that really counts).
. Extended objects will get brighter as objective size increases (again, subject to the exit pupil points above), but higher magnifications will spread the light out over a wider area of the retina and make it dimmer - rather lke spreading a fixed amount of butter over a larger slice of bread. So, for birding, it's a tradeoff.
Armed with this set of considerations, for me with my 6.5mm pupil size, 7x42 or 8x50 binoculars look the best bet - both have around 6mm exit pupils. 7x50s would have too large an exit pupil for me and 8x42s will be dimmer than the 7x42s. The 8x50s will yield a brighter image than the 7x42s, but will be bulkier to carry around. (If anyone's super-interested I can post some equations to work out relative brightnesses for different mag/objective combinations).
Finally, of course, quality of coatings is very important: There's no point in gathering all that light if only half of it gets through to the eyepiece!
Hope this helps,
Keith
For astronomy, increasing aperture or increasing magnification provides better views. Although spreading an extended object over more of the retina makes it dimmer, our eye/brain has an easier time perceiving objects covering more of the retina. Objects that are invisible at low magnifications can become visible when the magnification is increased. More stars are also visible at higher powers. Stars remain pinpoints, but the background sky is spread out more and dimmed.
Clear skies, Alan