A method of calibrating a digital pen (100) comprises the steps of: i) acquiring calibration data indicative of parameters indicative of an offset between a nib (108) of the pen (100) and a field of view (124) of a camera (114) whilst the pen is in use; and ii) using the calibration data to compensate for the offset in subsequently acquired pen usage data. A digital pen, digital paper and software for calibrating a digital pen and a network comprising a digital pen are also disclosed.