The invention concerns a method of processing pixel values comprising: during a first read phase, generating a first digital value as a function of pixel values by controlling, based on first and second control signals and a first set of increment rates, the rate that a first counter (220-i) is incremented; and during a second read phase, generating a second digital value as a function of pixel values by controlling, based on first and second control signals and a second set of increment rates, the rate that said first counter (220-i) is incremented, the first and second sets of increment rates each defining an increment rate for each of a plurality of states of the first and second control signals, wherein said first set of increment rates is different from said second set of increment rates.