A dermatoscope or endoscope inspection device which can be calibrated at high accuracy to be able to focus at a certain depth, e.g. below the top surface of the skin. A calibration pattern is provided or can be located at a reference viewing surface of an inspection device such as a dermatoscope or endoscope. It is important for a dermatoscope to know with the best accuracy available at what absolute depth below the top of the skin the device is focused. The dermatoscope or endoscope inspection device includes focusing means and by knowing a relationship between a digital driving level which shifts the focus position of the focusing means and the corresponding absolute change in focus depth, it is possible to know how deep the device is focused in absolute terms below the top of the skin.