The image pickup apparatus, a control method therefor, and a storage medium capable of maintaining detection accuracy of a line-of-sight position at the time of calibration even in a case where a focus controllable range changes due to a difference in a detachably mounted accessory. At a camera housing 1B, correction information about an individual difference of an eyeball is acquired by calibration based on a display position of an index in a finder thereof and a position of line-of-sight, of a user, detected by a line-of-sight detection circuit 201, the position of line-of-sight being poured into the index, and correction of the detected position of line-of-sight is performed using the correction information. Then, a position of line-of-sight, of the user, detected by the line-of-sight detection circuit 201, the line-of-sight position being poured into a through image is set to a focal frame. When one of the accessories is mounted thereon, a method for the calibration is changed according to a focus detection region acquired from the mounted accessory.