The Ideal BinocularsAll binoculars are characterized by two numbers: their power and the diameter of aperture or objective lens. For example: 7x35, 8x42, 10x50, 15x70, etc.
The first number indicates the power of the bino, i.e. how many times the bino will magnify the objects.
Some people might think that the magnification is the most important thing of binoculars. They might buy their first bino only because they have high magnification such as 15x or 20x, but they're wrong.
Many aspects should be considered when deciding to buy binoculars, such as: field of view, exit pupil, eye relief, weight, etc.
Binoculars with high magnification such as 15x70 or 20x80 tend to be heavy cause they have bulk design. Therefore, they won't be suited for travelling.
Big binoculars also have narrow field of view, which in turn, it won't be comfortable if you like to locate objects using those bino.
I have read some books (and websites) about astronomy and birding, and some writers come to the same conclusion. The ideal magnification is 8x, 9x, or 10x.
Anything below 7x will be too low, and anything higher than 10x will be difficult to hold them steady (unless you can afford buying an expensive Image Stabilizer binoculars).
I have browsed some websites just to look for the top quality binoculars from top manufacturers such as Leica, Swarovski, and Zeiss, and most of their expensive products are the mid size bino, such as 8x32, 8x42, and 10x42. I bet they have good reason in choosing these sizes.
Some aspects that should be considered when choosing binoculars:
If you want your binoculars to be handheld, go for magnification of 7x, 8x , 9x, or 10x. But if you can afford an Image Stabilizer binoculars, you can go for 14x or 15x.
Choose the bino that have field of view, at least 5°. But it would be better if you can get 8° or even more. It will make you easy to find target with wide field of view bino.
The minimum exit pupil is 3mm, but it would be preferred if you can get 4 or 5 mm of exit pupil.
If you want to use your bino for astronomy purposes, you should choose at least 30mm of objective lenses. The bigger, better. But no more than 60 mm, if you don't want to use tripod.
The bino are not too light, and not too heavy. In my opinion, a very light bino would be a bit difficult to hold steady. The same reason goes to the heavy ones.
So, what is the best all round binoculars?
Gary Seronik thinks that the best bino for astronomy are 10x50. Among his favorite are Canon 10x42 IS that cost more than US$ 1,000
Many birders might say that the best bino are 8.5x42 or 10x42 from top manufacturer such as Swarovski., that cost around US$ 2,000.
Many might say that Canon IS 10x30 would be perfect grab and go binoculars for day time use.