1) Magnification

When choosing your bird watching binoculars, it's important to get the right magnification for you. Remember, when you increase magnification, you also increase the amount of jiggle in the image. You'll want at least seven or eight power birding binoculars in order to see and identify the species you are observing. Less than that and you'll have difficulty picking out small details from far away. The most power you'll want is about 10 to 12. More than that and you will have real difficulty in holding them still. If you do want more power, consider a pair of stabilizing binoculars for birding. This technology will help you see more detail from farther away and will control the jiggle.

2) Sharpness of the image

One of the things that sets a great pair of birding binoculars apart from a mediocre pair is the sharpness of the image. Sharpness is determined by the quality of the lenses and the coatings used on them. The best binoculars will have multi-coated lenses. It costs more to grind lenses with greater precision, so expect to pay more for the best but the results will be well worth the price. Looking at a bird through a cheap pair of binoculars and looking at it through a good pair of birding binoculars is like night and day.

3) Durability

You're going to be toting your binoculars through nature. After all, that's where the birds are. Don't be alarmed if your birding binoculars get banged against a fence post or a tree. This could cause cheap or poorly made binoculars get out of alignment. This is very annoying as it will result in double images. Just as bad, if the tubes aren't sealed properly they can allow moisture inside and fog up or even get moldy. Some of the best birding binoculars are sealed with o-rings and filled with nitrogen gas.

Source：Monocular Telescope，Promotional Binoculars ，Gift & Promotional Binoculars