In some embodiments, a method of operating an image sensor supporting a low speed mode and a high speed mode includes: outputting a first set of output signals from a first pixel group to a first output line group by enabling, during a first period of the low speed mode, a first load circuit group connected to the first set of output signals; outputting a second set of output signals from a second pixel group to a second output line group by enabling, during a second period of the low speed mode different from the first period, a second load circuit group connected to the second set of output signals; and disabling the second load circuit group during at least a part of the first period.