An image pickup apparatus and a method thereof are provided. The apparatus includes a sensor array and an ADC array. The sensor array includes M×N sensor blocks SB(i,j). The sensor block includes P×Q image sensing elements Se(x,y). The ADC array is located at another side of the illuminated side of the sensor array. The ADC array includes M×N ADCs ADC(i,j). The ADC(i,j) coupled to the sensor block SB(i,j) obtains the image data Data(x,y) from the image sensing element Se(x,y) of the sensor block SB(i,j). The ADC(i,j) evaluates the gain G(x,y) based on the position of the image sensing element Se(x,y). The compensated image data Datacom(x,y) can be outputted and Datacom(x,y)=Data(x,y)×G(x,y). The image pickup apparatus could improve the optical shading problem.