This is the radius, in the imaging plane, of the smallest spot to which a collimated beam of light can be focused, which also corresponds to the size of smallest object that the lens can resolve. The size is proportional to wavelength, "λ", and thus, for example, blue light can be focused to a smaller spot than red light. If the lens is focusing a beam of light with a finite extent (e.g., a laser beam), the value of "D" corresponds to the diameter of the light beam, not the lens. Since the spatial resolution is inversely proportional to "D", this leads to the slightly surprising result that a wide beam of light may be focused to a smaller spot than a narrow one. This result is related to the Fourier properties of a lens.