1) Astronomy
For stargazing, light gathering is the most important factor. Choose a pair of binoculars with at least a 50mm aperture. our 7x50 model Military binoculars is easily hand-holdable and provides nice, wide-field views of starry swaths. The higher power 12x42 is also popular, and in fact is preferable to the 7x50 military binocular where skyglow is a problem. "Giant" binoculars of 70mm, 80mm or 100mm aperture will reveal fainter deep-sky objects and more subtle detail, but a tripod is recommended for a steady view. If you can afford the higher price (and a good tripod) and don't mind the extra bulk, you'll be rewarded with incredible views. (The item of giant binoculars, spotting scopes and telescopes are collecting, coming soon.)
The human eye on average has a 7mm pupil at night, so can be said that we look at the world through a pair of 1x7 binoculars! It follows then that even a small pair of 7x40 binoculars will show you a lot more detail than your unaided eyes can see. When Galileo first looked up through his primitive little telescope he only saw about as much as we can see through an average pair of binoculars today and he managed to create history. Binoculars are well worth remembering when it comes to looking at the night sky. With binoculars the moon is fascinating through all the lunar phases and even Jupiter and the four moons that Galileo saw is quite possible. We won't go into great depth about which binoculars are better than others for night viewing because there are too many variables to be able to say which is better. Suffice to say, any binoculars will show you a lot more than you can see without them and wherever possible try to steady them with either a tripod (use tripod bracket to attach the binoculars to any standard tripod) or just lean against a fence or post. For those keen to explore the night sky we recommend that you check our large range of telescopes. 
For more professional birdwatching, you may choose telescopes.
2) Birding
The most popular models for birdwatching are 7x40 military binoculars and 7x50 Floatable marine binocular. They're small and nimble, offer steady hand-held views, and have sufficient light grasp to provide bright, well-resolved images. If you plan to study birds at close range, look for binoculars with a near-focus distance of a few feet. Of course, other factors should play into your buying decision, such as eye relief for use with eyeglasses, optical coatings, and mechanical construction.
For more professional birdwatching, you may choose spotting scopes.

 

7x50 Military binoculars
7x40 Military binoculars