1. A method of correcting for pixel defects in an output of an image sensor, the method comprising: operating a preliminary filter to perform a preliminary filtering operation which operates on couplet defects wherein two adjacent pixels in a given image plane are defective, the preliminary filtering operation comprising replacing a signal value of a first pixel in the couplet with a corrected signal value and ignoring the other defective pixel in the couplet, with the ignoring comprising not using a signal value of the other defective pixel when replacing the signal value of the first pixel in the couplet; and operating a main filter to perform a main filtering operation on a signal resulting from the preliminary filtering operation.