Defocus is the mismatch between the receiving plane and the intended optical focus. Changing the plane changes the point spread function even when the optical lens is otherwise ideal.
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Defocus aberration is an optical distortion that occurs when light rays entering a lens do not converge at the intended focal point. This aberration typically results in images that appear blurred. It is primarily caused by the positioning of the lens relative to the image sensor or film plane, which can be affected by factors such as: 1. **Incorrect Focus**: If the subject is not perfectly in focus, the light rays will fail to converge at the correct point, leading to blurriness.