1. A calibration method for estimating a camera parameter representing a characteristic of imaging systems of a distance measuring device, each imaging system including a lens and an imaging area corresponding to the lens, and the calibration method comprising: capturing, using the distance measuring device, an image of a reference chart which (i) represents a geometric pattern in which elements are arranged and (ii) is positioned to have a predetermined positional relationship with the distance measuring device; modifying a luminance distribution affected by aberration of the lens in the captured image of the reference chart; calculating, as a feature point position, a gravity center position of each element in the captured image whose luminance distribution is modified; estimating the camera parameter using the feature point position calculated and an approximate actual gravity center position in the reference chart; and subtracting from a luminance value of each of the pixels, a removal value that is a value obtained by adding a predetermined offset value to a minimum luminance value within a corresponding one of calculation areas segmented for respective elements within the captured image, wherein the luminance distribution affected by the aberration of the lens is modified by subtracting the removal value from the luminance value of each pixel.