I recently bought a pair of Prostaff 7s binoculars to replace my car bins. Allbinos gave this model a good review. But I'm finding center focus to be a challenge at medium distances. For example, I was looking at a bird across the street and couldn't get a sharp focus. But looking at something farther away the center view seemed sharp. Is this a known issue or did I get a lemon? Trying to decide whether to exchange or return them.