Ask yourself some of the following questions. Your eyes will try to adjust to flaws in your optics and eye strain is a key factor in some of the lower end models if you bird for long periods of time. Here is where a lot of people make a mistake. The upper end binoculars can withstand a great deal more abuse than can the less expensive models. If you can't hold your binoculars steady you will not only be magnifying the bird but also the movement.
Some models are much easier than others to adjust the eye cups.
If you would like help with any of these questions or others, please call us during our shop hours. The first number refers to the power of the optics or how much closer the objects will appear. Powers from 7X-10X are recommended for birdwatching. Thus a 7X35 pair of binoculars would bring the object 7 times closer and the objective lens would be 35 mm in diameters. Well, the larger that number, the bigger and heavier the binoculars will be.