The Rodenstock Apo Sironar Digital HR was developed for special applications with extremely high resolution CCD chip cameras and CCD line scanner cameras with pixel sizes of <10 um such as can only be realised with smaller digital camera formats.  This lens utilises every technological possibility to get as close as possible to the absolute limit of diffraction-determined resolution.  Among other things, even the optical properties and the thickness of the CCD protective glass were taken into th equation of the optical correction.

The resolving power and lateral chromatic aberration have been optimised to ensure that the resulting lack of sharpness or the colour fringes do not amount to any more than a tiny fraction of the pixel size (which can no longer even be resolved).  As a result, even when the digital photos taken with the lens are enlarged to a maximum on the screen, absolutely no colour fringes are visible, unless colour fringes are added by the pixel structure of a one-shot camera back used or due to interpolation.

The resolving power of the Apo-Sironar digital HR is not only a little better for the working apertures of 8 to 11 recommended for other high-performance lenses.  You can rather see an increase in performance even with a higher aperture right up to the maximum aperture 4.  This increase is reflected in the very high brilliance and detail reproduction.  To ensure that this fantastic quality is not impaired by diffraction, the HR lenses should always be stopped down as little as possible.  This means that the depth of field should be increased for motifs extended in depth by using an optimum lens tilt of the camera for an overall sharp focus.

The larger apertures also reduce colour noise in the shadows.