Sharpness is pretty much psychological, and a lot of things are involved. Resolution is an important part of the sharpness issue, but high resolution can still result in the impression of poor sharpness. Contrast and edge effects help give the impression of high sharpness, and the scene itself is a big factor. Some pictures don't need great resolution to quite effective, or even to look sharp. (Certain portraits and scenes with large blocks as the principal subjects come to mind. Things that involve you emotionally, like many photojournalism shots, also cause you to ignore a lot of sharpness issues.) Other things demand high sharpness. In choosing a film, getting one that keeps a high MTF (modulation transfer function) at higher frequencies (line pairs per mm) is more important than just high resolution values in and of themselves. It tells you that contrast, which is important to the impression of sharpness, is maintained in the fine details. You will notice that some films have MTF values that go a little above 100 percent somewhere in the plots. That is evidence of enhanced edge effects, which also contribute to the impression of sharpness.