Why Rodenstock filters?
Filters can help produce better photos
Modern taking lenses produce sharp, high-contrast, true-color and distortion-free photographs. However, if ambient light is too bright for a low depth of field (e.g. for a high aperture) or for intentional motion blur (e.g. for a slow shutter speed), not even the best lens can help. It is no different if UV radiation falsifies colors and reduces contrast or if shiny surfaces produce irritating reflections. Or if colors are not converted into gray values in black-and-white im-ages, on film or digitally, as the photographer actually wanted. Only filters can solve this problem.
However, the quality differences are huge
If you have spent a lot of money for a good lens, you do not want to run the risk of image quality losses due to inferior filters. For the filter is just as much a part of the optical beam path as the lens elements.
- Irregularities in the filter lens surfaces
- Defective surface polishing and micro-scratches
- Inhomogeneity of the filter lens (varying refractive index)
- A lack of blacking on the filter lens edge
- A missing or insufficient coating
- Contamination (fingerprints, dust, drops of water)
This quality defects cause general or partial blur, loss of contrast due to scattered light or ghost images just like corresponding deficiencies in the lens elements.
An inferior filter frame can also have a negative effect with
- too great a design height (vignetting of wide-angle shots),
- insufficient blackening of the inside surfaces,
- imprecise thread cutting (stiff movement, jamming) and
- poor fit precision (risk of breakage, thermal expansion).
Filters therefore have to satisfy quality demands which are as high as those on lenses if they are not to impair the image quality.
Rodenstock quality filters ensure that the expected filter effects are implemented to perfection and that sharpness and contrast are maintained in full.
Light rays must remain parallel after passing through
To preclude the possibility of blur and loss of contrast, the direction of outgoing light must remain exactly paral-lel to that of incoming light at all points (see figure below):
- The glass must be homogeneous, that is it must have exactly the same refractive index over its total surface.
- Neither surface may be uneven or be tilted like a wedge toward the other; they must both be "planoparallel". Otherwise they will produce blur due to different light refraction
- The surfaces must be smooth and free of scratches. The polish must be equal to that of the surfaces of camera lens elements so as not to cause any contrast-reducing scattered light.
- The transmission of the glass should be as high as possible in the visible spectral range. Nor may it show any inclusions of foreign particles such as bubbles or striae. This would have negative effects on the image contrast and on the resolution.
- The mechanical strength should be high to allow a thin filterglass plate which is nevertheless break-resistant. Only then are shallow frames possible which avoid vignetting in wide-angle shots and which also still have a front thread.
All Rodenstock filters therefore have a very flat slimline frame. In order to allow a screw thread for fastening a lens hood or a protective cap, the glas is only 1.4 mm thick.
Rodenstock: 4 mm high slimline frame
Conventional 5 mm high filter frame
Brass frames for HR Digital, Skylight and Color Filters
The best material for filter frames is brass because it offers the best sliding properties when being screwed on or unscrewed as well as high mechanical strength (filters in black and yellow boxes). The frames of the Rodenstock filter lines Digital pro (in red boxes) and Digital Vario ND (in grey boxes) are made of the somewhat lighter aluminum because these filter lines were designed for the best possible price/performance ratio.
Blackened glass edges reduce irritating scattered light
The edges of all Rodenstock filter lenses are blackened so that no contrast-reducing reflections are caused there. Together with the excellent coating, this produces the best possible image contrast.
Although optical glass does not transmit 100% of light despite its high transmission, absorption does not play any role as filter glassplates are very thin. However, a smaller portion of light is reflected at any interface between air and glass. This reflected light reduces the useful brightness and can reduce contrast as scattered light if reflected a multiple of times. The reflection can be reduced for both sides of the filter glass together to around 2.5% for the medium range of the spectrum, with a slight rise to both sides, by vapor deposition of an exactly calculated, incredibly thin layer of a dielectric material of suitable refractive index, e.g. magnesium fluoride MgF2. A multilayer coating (MC = multi-coating) of several dielectric layers of calculated thicknesses and refractive indices reflects even less, that is below 0.2%. Its reflection curve is moreover particularly uniform over almost the whole spectral range (see figure below)
On the left, a Rodenstock filter HR Digital UV with a highly efficient super MC coating; on the right a non-coated UV blocking filter. The left hand filter with only 0.2% reflection practically no longer shows any mirror effect.
Water and dirt repellent coating
Filters remain clean longer and are easier to clean
Dirt on the front lens element or on a screwed on filter reduces the contrast. Fingerprints and water droplets have a soft-focus effect. Careful cleaning is therefore required regularly for good sharpness. A high-quality, scratchresistant UV blocking filter is recommended since a scratched front lens element can make the lens, which is expensive in comparison with the filter, impossible to use. A part from clear far view with less bluish haze und improved sharpness, it has no visible effect on the image and can therefore remain on the lens at all times. The coating of all Rodenstock filters is more scratch-resistant than the hard glass. The filters of the HR Digital line with "super MC" have a further coating on top of the MC coating which is even harder and has an additional beneficial property: It is repellent to water and oil. This means that water does not wet the glass, but rather forms pearl-like drops as on the lotus blossom famous for this effect.
Water forms spheres and rolls off
Water adheres to the filter glass
The UV blocking filter HR Digital UV "super MC" is in particular the ideal colorless lens protection filter. It protects the front lens element against dirt and scratches.