Defocusing blur
Sensors and processing for 3D passive imaging

Outside the zone of sharpness, objects seen by an imager appear with a defocusing blur (DFD) that increases with the distance between the object and the plane of focus (figure 12). In this diagram, the plane of focus passes through the point of convergence of the green rays: objects located in this plane appear sharp in the image. In contrast, an object located beyond the plane of focus produces a blurred image, as the red rays do not converge at a point on the image plane. The width of the blur, noted ε in figure 12, can be estimated as a function of the depth Z by geometric considerations and using the conjugation relation for thin lenses: