What is a CENTERFILTER?
Wide-angle lenses are characterised by decreasing light intensity towards the edge with increasing aperture angle, which makes the corners appear darker and is unwelcome in many cases. In conventional silver photography concentric gradient filters referred to as CENTERFILTERs were developed. These are filters with a higher density in the centre of the image and increasing transmission towards the edge.
CENTERFILTERs for analogue photography have to be mounted on the lens when the film is exposed. They correct the brightness reduction by a fixed value – usually 1.5 or 2 aperture stops. ANALOGUE CENTERFILTERs consist of a glass plate that is coated with a special grading in a complex production process and a filter support that positions the plate at an exactly calculated position in front of the first lens.
Disadvantages of an ANALOGUE CENTERFILTER:
- Possible vignetting at ultra-wide angles > 100°
- Several filters may be required, e.g. an additional polarising filter
- Risk of reflexes, reflections and “ghost images” on the filter surface under unfavourable light conditions
- Loss of brightness during focusing
- Symmetry interference with high-resolution lens systems
Why use a DIGITAL CENTERFILTER?
In contrast to analogue photography, in digital photography each image is stored in a file and can be (post-)processed using suitable software. If the brightness reduction of the lens over the whole image circle is known, it can be corrected – provided the edges of the image were sufficiently exposed – with the aid of special software, the DIGITAL CENTERFILTER. A DIGITAL CENTERFILTER can also be used for correcting the edge brightness reduction in analogue images after they have been digitised for processing.
Advantages of a DIGITAL CENTERFILTER:
- The filter is applied after the image has been taken, so that the full brightness of the lens is retained
- Virtually 100% edge shading correction, even with offsets (offset values xy can be entered in the software)
- Continuously variable correction of the exposure value – aperture 0.1, 0.5 or 1.5, anything is possible.
- A correction can be discarded and repeated, if necessary
- No additional glass in front of the lens system that may cause reflections and reduce the resolution
- No vignetting of fitting components
In order to enable users to create optimum images, Schneider Kreuznach and Nurizon Software have developed a DIGITAL CENTERFILTER as a plug-in for Photoshop® (from version CS2).
Schneider Kreuznach – Nurizon DIGITAL CENTERFILTERs are based on the exact specification for the corresponding Schneider Kreuznach lens and therefore guarantee optimum correction of the brightness reduction in wide-angle images.
The software for the Photoshop® plug-in and the lens-specific file for the DIGITAL CENTERFILTER will be included in the price of each Schneider Kreuznach wide-angle lens from May 2010.
Further files for using a DIGITAL CENTERFILTER for other focal lengths are available as options.