Binocular sizes look like a continuum and behave like a set of distinct tools. This page runs through the common ones with the exit pupil each gives, which is the number that predicts how a pair will actually behave.
The pocket sizes
8x21 gives a 2.6mm exit pupil and 8x25 gives 3.1mm. Both are daylight instruments: bright and sharp at midday, dim at dusk, useless on the night sky. The 25mm is worth the small extra bulk over the 21mm for about forty per cent more light.
The high magnification compacts
20x50 gives 2.5mm and 20x60 gives 3mm. These promise telescope-like reach in a binocular body and deliver a dim, shaky view handheld. Mounted on a tripod they are genuinely useful, and that is how they should be bought.
The night sky sizes
8x56 gives seven millimetres, matching a young dark adapted pupil, and is a specialist low light instrument used by hunters and astronomers. 15x70 gives 4.7mm and needs a mount. Both are heavy and both show substantially more than a 50mm pair on faint objects.
Reading a size before you buy it
Every one of these configurations can be judged before you handle it, by dividing the second number by the first. Anything above five millimetres will work at night; anything below three is a daylight instrument whatever the box says. That single division predicts more about how a pair will feel at dusk than the brand, the price or the coatings do, and it takes about two seconds.
Questions people ask about 8x25 binoculars
Are 8x21 binoculars any good?
In daylight, yes. At dusk and at night, no: the 2.6mm exit pupil simply does not deliver enough light.
Are 20x50 binoculars good?
On a tripod, useful. Handheld, the shake at twenty times removes most of the detail, and the 2.5mm exit pupil makes the view dim.
What size binoculars for low light?
An exit pupil of five millimetres or more, so 10x50, 7x50 or 8x56 depending on how much magnification and weight you want.