An image pickup apparatus that is capable of obtaining a defocus amount for focus detection with high accuracy at high speed with a simple configuration even in a time of image stabilization. The image pickup apparatus including a memory device that stores a set of instructions, and at least one processor that executes the set of instructions to obtain optical parameters about an image pickup optical system and an image sensor as reference information, correct the reference information based on a relative moving amount of the image sensor with respect to an optical axis of the image pickup optical system, obtain a control parameter corresponding to corrected reference information by referring to an information data set that stores the control parameter used for finding a defocus amount for focus detection in association with the reference information, and find the defocus amount based on the control parameter obtained.