I am debating about moving from my D90 to the D600. However, I came up with some foruns commenting about how small is the area the AF sensor in the D600 can cover, and that this may be a real problem. Since I don't have experience with anything beyond the D90, how much this can really be a problem? I most of the time take pictures of my kids around the house, portraits and in school presentations. Would that small area be a problem for such situations?