Focus on it and then move the binoculars slightly so the subject that was in sharp focus is now at a far edge of the binoculars. Check to make sure if it is as sharp as it was when it was in the center of the binoculars. Remember any binoculars may exhibit some of these problems to some degree so your purpose here is to compare one pair against the other to determine what is best for you. We have found that binocular buyers feel more comfortable when they understand what the model numbers mean and how to focus your binoculars prior to shopping for them. But lastly, we suggest that you check to see what kind of warranty comes with the binoculars.
Photographic lenses are better than lesser quality optics.
For example, roof prism binoculars are generally more expensive than porro prism style binoculars of the same overall quality due to their more complex optical and mechanical design. If all else is equal, 7x50s will cost more than 7x35s. Again, if all else is equal, 10x50s will be more expensive than 7x50s. Let's look at these factors one at a time. This insures the brightest, highest contrast, and sharpest image possible.