The invention provides a lens apparatus, an image capturing apparatus, a camera system, a determination method and a storage medium. The lens apparatus is detachably attachable to an image capturing apparatus including a focus detection unit having a focus detection optical system and performing focus detection process, and a controller controlling driving of a focusing lens unit based on a resultof the focus detection process, via an extender including an extender optical system, includes an image capturing optical system including the focusing lens unit, and a determination unit determininga first correction value used for correcting the result of the focus detection process. When the lens apparatus is attached to the image capturing apparatus via the extender, the determination unit determines the first correction value using first information regarding a diameter of a pupil of the focus detection optical system and second information regarding a diameter of a pupil of an opticalsystem including the image capturing optical system and the extender optical system.