A few things about binocular, In no particular order.
The first number is the aperture the second number is the magnification.
If you divide the aperture by the magnification you will get the exit pupil of the binocular. If the exit pupil matched a fully dilated pupil of your eye (7mm) then you will get maximum brightness. This being said a 35 x 5, 50 x 7 and 70 x 10 will all appear to be the same brightness. If you are going to view when light gathering is important this is what you should look for.
Wide angle binoculars are generally easier to use to find birds that are on the move. The downside is wide angle binoculars tend to be soft at the edges. So wide angle are the best bet for finding small birds that are on the fly.
Consider how close the binoculars will focus if you intend to observe birds at a feeder. You will need binoculars that will focus at the feeder distances. I find close focusing good for observing bug.
Once you reach a certain dollar value in binocular you don’t get much better image quality with any more money but you get binoculars that are more comfortable to use over longer periods of time.
There are generally two typed of binoculars the more common is Porro Prism Binoculars and they come full sealed or not sealed. These are the binocular that have the bumps on them. The second type are Roof Prism Binoculars. They look like two telescopes joined together. They are more stylish and more expensive.
You should be able to focus from far to near with ease and not have to spin that center wheel a billion turns. One eyepiece should be adjustable independent of the other.
Eye relief is the distance you hold the binoculars away from your eye and still get full view. This should be sufficient enough to place glasses between you and the binoculars, You may not now need glasses now but what is in the future is unknown. I you don’t have astigmatism you won’t have to wear glasses with your binoculars but you may choose to.
All binoculars are not the same and they must be tested before buying. I picked up a pair of Nikons for my wife and drove the salesman batty as I testing all twelve he had in stock. Nine were good, two were marginal and one was superior. Remember these were all the same model, same price, and same manufacturer.
The first thing to look for is good collimation. That is do both tubes look at the same object equally? Take the outside and look at a very distant bird, on that is a spot. If you look at this bird and see two spots they are not collimated
Next is barrel and pincushion distortion. All binocular have one of these. Take it on you want to buy outside and look a distant straight line. While telephone or power lines are not truly straight you can find an area where they are not bad. Look at they through the binoculars so they are parallel to the ground. In the middle you should see not distortion but a straight line. As you move the line up and down in the binocular field of view it will distort near the edges. It is the degree of distortion that determines the which binocular to buy. Less if good. A wide angle binocular will generally show more distortion that a normal binocular but this is to be expected.
Finally make sure they are multi coated.
