An image system comprises a light source, an image sensing device, and a computing apparatus. The light source is configured to illuminate an object comprising at least one portion. The image sensing device is configured to generate a picture comprising an image. The image is produced by the object and comprises at least one part corresponding to the at least one portion of the object. The computing apparatus is configured to determine an intensity value representing the at least one part and to determine at least one distance between the at least one portion and the image sensing device using the intensity value and a dimension of the at least one part of the image.