1. An optical apparatus including a lens system having a focus lens and an image pickup element which converts an image formed by the lens system into an image signal, the apparatus comprising: a control unit configured to control operation of the apparatus; an auto-focus unit configured to move the focus lens to bring the image into an in-focus state; a displacement detection unit configured to detect a displacement of the optical apparatus in the direction of the optical axis of the lens system; and a focus-range setting unit configured to calculate an acceptable range of detected displacement within which it is assumed that the image remains in an in-focus state, wherein the control unit is operable to capture one or more images in a given time period from the pickup element when the detected displacement is within said acceptable range.