Image capturing apparatus and control method thereof

A method of controlling an image capturing apparatus including an image sensor including a first semiconductor substrate on which a plurality of photodiodes are arranged, a second semiconductor substrate on which a plurality of storage elements configured to store pixel signals generated by the plurality of photodiodes are arranged, and a plurality of connection units configured to electrically connect the plurality of photodiodes and the plurality of storage elements, the method comprising:

• a first transfer step of transferring, to the storage elements, pixel signals of a first pixel group that have been accumulated in every predetermined frame;
• a first readout step of reading out, for every frame, the pixel signals of the first pixel group that have been stored in the storage elements;
• a second transfer step of transferring, to the storage elements, pixel signals of a second pixel group different from the first pixel group that have been accumulated at a predetermined timing;
• and
• a second readout step of reading out some of the pixel signals of the second pixel group that have been stored in the storage elements,
• wherein an image of one frame is generated by composing the pixel signals of the second pixel group that have been read out through a plurality of frame periods, and the pixel signals of the first pixel group that have been accumulated simultaneously with the pixel signals of the second pixel group.