An integrated-circuit image sensor generates, as constituent reference voltages of a first voltage ramp, a first sequence of linearly related reference voltages followed by a second sequence of exponentially related reference voltages. The integrated-circuit image sensor compares the constituent reference voltages of the first voltage ramp with a first signal level representative of photocharge integrated within a pixel of the image sensor to identify a first reference voltage of the constituent reference voltages that is exceeded by the first signal level.