Another limitation comes with optical diffraction. Even in ideal optics the achievable resolution of a camera system is limited. The Sparrow criterion suggests3 that there is no gain in resolution below a critical pixel size of . For our example of and , we obtain . Achieving this limit, however, is challenging, especially in the off-axis field, and leads to expensive optics. A further decrease in aperture requires a dramatic increase of the technological efforts and smaller tolerances for optics manufacturers.