Accessories: Center filter
For critical shots (e.g. with motifs of uniform brightness) taken with lenses with an extremely large image angle, both in analog and in digital photography, the physically inevitable light fall-off can be an irritating element. With a specific software like Photoshop® digital shots can be brightened up, but only up to a certain level. At the margin or corners already existing dark objects may lose sharpness and depth (because of black becoming dark gray) and it can lead to visible image noise. The Rodenstock center filters largely eliminate light fall-off and therefore should be always used if the image circle of wide-angle lenses (HR Digaron-S 23 mm and 28 mm) is used right up to the vicinity of the circumference. The Rodenstock center filters designed especially for these lenses will be available soon.
Due to an “optical trick” (as already mentioned in the lens description), the light fall-off of the other Rodenstock wide-angle lenses is less than that of common technical camera lenses with a similar image angle. This means that no center filter is required.
Center filters are concentric graduated neutral gray filters whose density decreases from the center up to the transparent rim. From the recommended working aperture on the fall in density of the center filter compensates for most of the light fall-off to the image edge. So the light fall-off is not stronger than for lenses with a longer focal length and does not represent a disturbing element any longer. Due to the light fall-off at the image center the exposure time must be corrected according to the table.
Reflection reduced digital center filters
New color-neutral center filters for compensating the physically induced significant falloff in ilumination of super wide-angle lenses.
Conventional center filters for wide-angle lenses with image angles larger than 100° reduce the fall-off in illumination of 2 to 3 f-stops by reflection and absorption of their neutral density coating that gradually gets darker to the center. But this reflection produces stray light. The new Rodenstock center filters work only by absorption, hence they provide more brilliant and neutral pictures.