Although this sentiment surely has wider philosophical applications, many imagers are familiar with the feeling. You have spent 5 or 10 minutes (or more) working on focus and it "looks pretty good". But is your focus really as good as it can get and how can you be sure it is? This can be a major problem with many techniques and few imagers are happy with "Just OK focus" or "I think it's right". The goal of the next few sections is to help you to be sure that you are truly focussed.