Pixels within an image sensor pixel array are sampled by corresponding conditional read circuitry. A zero pixel value is outputted for each pixel associated with a sample less than a conversion threshold, and a saturated pixel value is outputted for each pixel associated with a sample greater than or equal to a saturation threshold. Samples greater than or equal to the conversion threshold and less than the saturation threshold are converted by an ADC, and a converted pixel value is output for each associated above threshold pixel. The ADC (along with any corresponding amplifiers) are powered on for a variable period depending on the number of pixels needing conversion during the conversion of such samples during a read period, and are powered off for the remainder of the read period.