MorganS:
0.25mm accuracy is ridiculous. Unintentional changes in pressure will spread your fingertip out by more than that much.
It's not ridiculous if you want to detect slight movement from side to side. Yes, maybe absolute finger position to 0.25mm is "ridiculous", but relative position is a different matter. If sliding the finger is to control, say, frequency, then such small resolution might be needed to get a smooth frequency change -- the ear can detect very minute frequency changes.