From the point of view of diffraction theory of optical image formation, each frequency term in the Fourier summation is regarded as a single information carrying element. The amount of information transmitted to the image space is limited by the size of the optical pupil, and is proportional to its area, irrespective of its shape or any phase modifying device which may be present at the pupil. A quantity called "information density" is introduced. The relation between resolving power and the amount of information carried through an optical system is discussed.