This relates to my post in the "40x60 from 35mm" thread. When we look at things from normal viewing distances, the subjective evaluation of definition has a lot to do with whether or not we have the impression there is as much detail as we think should be there. This is complicated by the visual/mental tendency to "fill in" missing detail. And this tendency is even further complicated when the subject matter is familiar because we know what should be there, which makes it that much easier to fill in whatever detail might actually be unresolved in the image.