This can be explained as follows. Each point in the image can be considered to be illuminated by a finite area in the object. The size of this area is determined by the diffraction-limited resolution of the lens which is given by the Airy disk whose diameter is 2.4λu/D, where λ is the wavelength of the light, "u" is the distance between the object and the lens, and "D" is the diameter of the lens aperture. (This is a simplified model of diffraction-limited imaging.)