A method of measuring working distance between a handheld digital device and eyes of a user, including capturing an image of at least eyes of a user via an onboard camera of the handheld digital device while the user is viewing a display of the handheld digital device and comparing an apparent angular size of a structure of the eyes or face of the user to a previously captured image of the structure of the eyes or the face that was taken in the presence of an object of known size. The method further includes calculating a working distance based on the apparent angular size of the structure of the eyes or the face; and saving at least the working distance to memory or reporting out the calculated working distance on the display. A handheld digital device programmed with an algorithm to perform the method is also included.