1. An apparatus for calculating the position of an object, the apparatus being suitable for an optical touch system and being provided for determining a representative position of the object in a sensed image, the apparatus comprising: an image sensor; and a processing circuit electrically coupled to the image sensor, the processing circuit acquiring a sensed image containing an image of the object through the image sensor, and the processing circuit obtaining brightness value of at least one pixel in each pixel column of the sensed image to obtain a first brightness distribution profile, the processing circuit comparing the first brightness distribution profile with a second brightness distribution profile obtained from a background image, so as to select a plurality of column pixel brightness values in the first brightness distribution profile as an imaging range of the object, the processing circuit selecting column pixel brightness value corresponding to at least one dot from a curve segment of the imaging range in the second brightness distribution profile, so as to regard the selected column pixel brightness value as a brightness reference value, the processing circuit further calculating the brightness difference between the brightness reference value and the column pixel brightness value corresponding to each dot of the curve segment of the imaging range in the first brightness distribution profile, so as to calculate a gravity of the imaging range and to regard the gravity as a representative position of the object in the sensed image.