1. A photoelectric conversion apparatus comprising:
a pixel array including a plurality of pixels arranged in a plurality of columns, each of the plurality of pixels including a photoelectric conversion unit, and each of the plurality of pixels further including a digital-signal output unit configured to output a digital signal based on an output of the photoelectric conversion unit;
bus lines each arranged in correspondence with one of the plurality of columns, and each configured to output the digital signal from the digital-signal output unit in a pixel of the plurality of pixels; and
a control unit configured to control the digital-signal output unit to switch number of bits of the digital signal.