Seven by fifty was the standard marine and astronomical binocular for most of the twentieth century, and it is still the right answer for a specific set of people. The reason is the seven millimetre exit pupil, which is both its strength and its limitation.
The exit pupil argument
Fifty divided by seven is 7.1mm. A fully dark adapted young eye opens to about seven millimetres, so a 7x50 delivers every photon it gathers into the eye. As people age the maximum pupil shrinks, often to five millimetres by middle age, at which point part of the light cone is blocked by the iris and some of the aperture is wasted.
What the lower magnification buys
A wider field, typically seven degrees or more, which makes sweeping the Milky Way and finding objects effortless. A brighter image, because the same light covers a smaller area. And far less visible shake, which for many people means more detail seen despite the lower power.
Who should buy which
Younger observers and anyone who values a wide steady field: 7x50. Older observers and anyone who wants more detail on clusters and the moon: 10x50, which puts a five millimetre cone into an eye that can use it. Both are excellent; neither is a compromise.
Questions people ask about binoculars
Are 7x50 binoculars good for astronomy?
Very. They give the widest, brightest, steadiest view of any handheld 50mm configuration, and they were the traditional astronomical and marine standard.
What does 7x50 mean?
Seven times magnification through 50mm objectives, giving an exit pupil of about 7.1 millimetres.
Are 7x50 or 10x50 better for the night sky?
Seven by fifty for younger eyes and for sweeping; ten by fifty for detail and for eyes whose pupils no longer open to seven millimetres.