Defocus aberration is a type of aberration in which an image is simply out of focus. This type of aberration commonly occurs when using optical microscopes, telescopes, or binoculars.
Optically, defocus refers to a translation of the focus along the optical axis away from the detection surface. In general, defocus reduces the sharpness and contrast of the image. What should be sharp, high-contrast edges in a scene become gradual transitions.
Fine detail in the scene is blurred or even becomes invisible. Nearly all image-forming optical devices incorporate some form of focus adjustment to minimize defocus and maximize image quality.