Lenses for Digital Professional Photography
Accessories: Centre filters
For critical shots (e.g. with areas of uniform brightness towards the image corners) the physically inevitable light fall-off according to the “cos 4 law” may be visible. This is also known as "lens fall-off".
Digital photos can be "corrected" or lightened at the edges and corners with imaging software like Photoshop® or Capture One Pro. This correction method may not work effectively if the subject matter at the edges of the captured frame are dark or under exposed. These areas that are under exposed and may not show the correct structure, colour and depth. If they were lightened or adjusted using software, the images may show digital noise or incorrect colour.
The light fall-off can be eliminated by using the neutral grey Rodenstock center filters available for the HR Digaron-S 23 mm and 28 mm as well as for the HR Digaron-W 32 mm. A centre filter should always be used if the image circle of the lens is used right up to the vicinity of the circumference.
Rodenstock center filters are concentric graduated neutral grey filters whose density decreases from the centre out to the transparent rim. The fall in density largely compensates for the light fall-off to the image edge from the recommended working f-stop. The remaining light fall-off is similar to that of lenses with longer focal lengths and does not appear as severe. Because of the density of the centre filter, the exposure must be corrected either by extended exposure time or by a wider aperture.
Rodenstock wide-angle lenses from 35mm focal length onwards do not require a centre filter. By utilising an “optical trick” (tilting and increasing entrance pupil for larger incident angles with the optical axis), the light fall-off of the lens is able be reduced almost to a level of lenses with a standard focal length, hence no centre filter is required.