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Since the beginning of the ALPA Medium Format era, lenses from Schneider Kreuznach were part of the ALPA photographic plattform. Being mostly true symmetrical constructions, these lenses featured rather large image circles and a stable performance from infinity down to a scale of 1:4. Center Filters cured the often very prominent vignetting / edge light fall-off. ALPA used / uses the brands Helvetar and Switar for dedicated lenses. The Helvetar 5.6/48 mm is still one of the most flexible lenses for analog and digital use and the lens with the highest production numbers. It was retired with the arrival of the extraordinary Helvetar 5.6/43 mm lens.
Helvetar 5.6/28 mm
Switar 5.6/36 mm
Helvetar 5.6/43 mm
Helvetar 5.6/48 mm
Helvetar 5.6/60 mm
Helvetar 5.6/75 mm
Helvetar 5.6/120 mm Apsheric
Specific photographic applications need an extremely differentiated scale focusing - e.g., when using a laser distance meter. ALPA offers with the ALPA HPF rings a unique and retrofittable solution for these requirements, which is - in contrast to other offerings - truly usable in freehand operation, which can also be used in the field without complicated translation work. The rings are available in easy to read and comprehensible meter and feet scales.
Every scale features engraved distance readings from infinity, the first five degrees, and then consecutively every five degrees up to 270 degrees. The single degrees in between are marked. If you need the full resolution, you can download the detailed distance tables for the lenses. The table also offers the corresponding metering when using the ALPA macro adapters.
For even better handling, we added the removable focusing knobs to every HPF ring.
When working with ultra-wide-angle lenses, the falloff of illumination towards the corners is sometimes disturbing. For this reason, concentrically graduated Center Filters have been designed to compensate for this vignetting. These filters have transparency that increases gradually from the center to the edges of the filter, where it reaches full transparency. The transmission is independent of the wavelength of the visible spectrum.
To avoid overexposure and consider the film emulsions' exposure latitude in the analog area, the optic manufacturers do not fully compensate for the lens's vignetting. This working hypothesis was adapted to the digital world. Specification 1.5, 2.5, etc. indicates the extension factor for the exposure.
As Center Filters must be on par with the lens quality, they are manufactured at the same level as optics. The price reflects this level of complexity.
The brightness drop / vignetting of digital backs can also be compensated by a digital correction function. This process is achieved by image analysis or, in the more complex and more accurate case, by a correction image with digital analysis. The latter variant (LCC Lens Cast Calibration, Scene Calibration) analyzes the edge falloff and color shifts utilizing a separate neutral image taken with an opaque white disc.
Since this process has to be offset against the image(s), the correction image is taken with the same aperture and shift adjustment as the original. This correction benefits from the existing dynamic range. For an optimal result, it may therefore be necessary to combine both techniques.
