A CMOS image sensor comprises an array of pixels. A column of the pixel array is coupled to a readout column. The readout column is couple to a readout circuitry (RC) that reads out image data from the pixel array. The RC comprises a sampling switch which is coupled to a 1-column successive approximation register (SAR) analog-to-digital converter (ADC). The 1-column SAR ADC comprises a differential comparator, a local SAR control, and a digital-to-analog converter (DAC). The sampling switch is coupled between a readout column and a non-inverting input of the differential comparator. An image readout method reads one pixel with two conversions through the RC. The RC is operated by the local SAR control to set the DAC based on comparator output, and upon which a reset digital value is obtained and stored. An overall reduced algorithm calculation is achieved herein.