A distance calculation method and an imaging device that can be applied to a digital camera. The method includes calculating a distance between a point corresponding to an optical axis of a lens sub-system of the imaging device and an input point, and using the calculated distance to correct defects such as shading that occur in formed images due to peripheral light fall-off in the lens sub-system. The distance is approximated by a polygon having five or more sides, preferably having 8 or 16 sides. The distance is calculated by a disclosed function having constant coefficients, so that distances can be calculated by simple hardware.