1. A method of correcting an image signal generated by a charge coupled device (CCD) image sensor of an imaging system, the charge coupled device (CCD) image sensor comprising a plurality of effective pixels for producing the image signal and a plurality of optical black (OB) pixels for determining a dark reference of the image signal, the imaging system comprising a memory for storing a plurality of gamma correction curves, each of which has a respective correction factor to increase contrast in a dark portion of the image signal, the method comprising: measuring a plurality of gray scale values of the plurality of effective pixels and the plurality of optical black (OB) pixels in the CCD image sensor; estimating a contrast level of an object scene to be imaged using the plurality of measured gray scale values; and correcting the image signal using a corresponding gamma correction curve depending on the estimated contrast level of the object scene, wherein the estimating of the contrast level of the object scene comprises: determining whether a smear effect induced in the CCD image sensor is acceptable; counting an amount of effective pixels that are in a dark portion of the image signal, if the smear effect is acceptable, and estimating a contrast level of the object scene using the amount of the effective pixels in the dark portion of the image signal, if the smear effect is acceptable, and estimating the contrast level of the object scene according to the level of the smear effect, if the smear effect is not acceptable.