1. A method of performing an absolute length calibration for an object ( 20 ) located with or covered by a camera device ( 15 ) in a mirror ( 10 ), the method comprising the steps of: determining a length of the camera device ( 15 ) based on the type of the camera device ( 15 ), capturing an image, using the camera device ( 15 ), of the camera device ( 15 ) and the object ( 20 ), measuring a length of the image of the object ( 20 ) and measuring the length of the image of the camera device ( 15 ) from the captured image, deriving a calibration function from the determined length of the camera device ( 15 ) and the measured length of the image of the camera device ( 15 ), and using the measured length of the image of the object ( 20 ) and the calibration function to calibrate the length of the object ( 20 ).