A photo sensor includes a plurality of pixel blocks, each including one or more anchor pixels and one or more non-anchor pixels. The anchor pixels produce first sensor signals and the non-anchor pixels produce second sensor signals. An amplifier circuit amplifies the first and second sensor signals. A variable bit-depth analog to digital converter (ADC) circuit quantizes amplified versions of the first sensor signals into first digitized sensor signals with a first bit-depth. The ADC circuit also quantizes amplified versions of the second sensor signals into second digitized sensor signals with a second bit-depth that is lower than the first bit depth. The second bit-depth may be selected based on anchor pixel statistics derived from the one or more first digitized sensor signals.