A solid state image sensor, comprising: a first wafer having a back side and a front side, the first wafer including a backside illumination sensor array comprising an array of pixels, each of the pixels comprising a photosensor located on a back side of the first wafer, each of the pixels is configured to output a first value indicative of the amount of light received by the corresponding photosensor; and a second wafer having a back side and a front side, the second wafer comprising an array of readout circuits, each of the readout circuits being configured to output a readout signal indicative of an amount of light received by a corresponding one of the pixels, each of the readout circuits comprising a comparator and a counter, each of the comparators is configured to: i) receive the first value from a corresponding one of the pixels, ii) compare the received first value to a photosensor threshold, and iii) output a second value indicative of whether the first value is greater than the photosensor threshold, a third wafer having a back side and a front side, the third wafer comprising a digital signal processor formed as an array corresponding to the array of readout circuits of the second wafer; wherein the first and second wafers are stacked using through silicon via bonding between the front side of the first wafer and the back side of the second wafer, the second and third wafers are stacked using fusion bonding between the front side of the second wafer and the front side of the third wafer; wherein each of the counters is configured to: i) receive the second value from a corresponding one of the comparators and ii) increment in response to the received second value being indicative of the first value being greater than the photosensor threshold, wherein each of the readout circuits is configured to generate the readout signal based on a value stored in the corresponding counter and a remainder stored in the corresponding pixel, and wherein each of the pixels is further configured to be reset in response to receiving an amount of light that is greater than the photosensor threshold. a first wafer having a back side and a front side, the first wafer including a backside illumination sensor array comprising an array of pixels, each of the pixels comprising a photosensor located on a back side of the first wafer, each of the pixels is configured to output a first value indicative of the amount of light received by the corresponding photosensor; and a second wafer having a back side and a front side, the second wafer comprising an array of readout circuits, each of the readout circuits being configured to output a readout signal indicative of an amount of light received by a corresponding one of the pixels, each of the readout circuits comprising a comparator and a counter, each of the comparators is configured to: i) receive the first value from a corresponding one of the pixels, ii) compare the received first value to a photosensor threshold, and iii) output a second value indicative of whether the first value is greater than the photosensor threshold, a third wafer having a back side and a front side, the third wafer comprising a digital signal processor formed as an array corresponding to the array of readout circuits of the second wafer; wherein the first and second wafers are stacked using through silicon via bonding between the front side of the first wafer and the back side of the second wafer, the second and third wafers are stacked using fusion bonding between the front side of the second wafer and the front side of the third wafer; wherein each of the counters is configured to: i) receive the second value from a corresponding one of the comparators and ii) increment in response to the received second value being indicative of the first value being greater than the photosensor threshold, wherein each of the readout circuits is configured to generate the readout signal based on a value stored in the corresponding counter and a remainder stored in the corresponding pixel, and wherein each of the pixels is further configured to be reset in response to receiving an amount of light that is greater than the photosensor threshold.