A polarizer for the Hasselblad fisheye lens

General remarks

Polarization in the landscape is produced predominantly by Rayleigh scattering from molecules in the atmosphere and by specular reflection from dielectric (non-metallic) surfaces. The light entering the camera is polarized with the electric vector perpendicular to the plane containing the source (sun), the scatterer/reflector and the camera. The degree of polarization is greatest for scattering at right angles and for reflection through twice the Brewster angle (ie, about 106 degrees for water). The degree of polarization decreases for smaller and larger angles. Models of the polarized intensity and the fractional polarization for Rayleigh scattering a clear sky with 80% maximum polarization are shown in these 'Mathematica' plots. This means that for a fisheye lens like the 30mm Distagon, the degree of polarization from the clear sky and from the landscape varies dramatically across the field of view (FOV, diagonal 180 deg, edge 112 deg for this lens). Consequently, a polarizer has a very different effect from one on a lens of normal focal length. It is capable of painting a large contrast in brightness and colour across a blue sky: enhancing the visibility of clouds in some directions and decreasing it in others.

Practicalities

The polarizer made for me by Hasselblad was supplied separate from the metal ring and so it is possible to mount it in the lens at any angle. I have marked on the edge of the filter the axis which transmits the E-vector and I usually mount it with this in either the horizontal or the vertical direction. Once mounted, I check the exact orientation by looking through a polaroid strip with a known orientation. Small adjustments can be made by loosening the locking ring and twisting the filter with a clean finger.

With a low sun, mounting the filter with the E-vector transmission horizontal produces dramatic effects. This means that a picture taken of the northern or southern horizon at sunrise or sunset will produce a vertical dark band in a clear sky as is demonstrated in the following pictures taken on the Island of Lundy during July 1998.

Lundy sunset looking south (inverted)

Zenith

Lundy sunset looking north

Here is a pdf file containing these images and some explanation.

A vertically mounted filter, however, reduces reflections from horizontal surfaces like still water.

As for a normal polarizer, I generally use an exposure correction of 1.5 - 2 stops.

Applications

I have only just begun to explore the possibilities but it is clear that one of the most striking applications is photography of scenes with a large solid angle of blue sky. Even with the sun within the FOV, regions of right angle scattering are included and so the range of brightness in the sky is large (a clear sky can be close to 80% polarized at right angles to the sun). This contrast range can be used to paint structure in compositions which would otherwise be rather flat. The polarizer also increases colour saturation in the landscape and reduces distant haze.

Keck I and II telescopes on Mauna Kea, Hawaii, looking west

Taubenberg, Bayern, looking east

A less conventional application is in the 'keyhole' view of the bay and cliffs in Hawaii. Although taken towards the sun, the horizontal polarizer has dramatically increased the colour saturation of the foliage on the left and (especially) the right of the picture.

Pololu valley, Hawaii

Recommendations for filter construction

The unmounted filter works well but it is difficult to insert and orient in the field. I think a better solution might be to mount the filter in the ring with a fiducial mark indicating the orientation of the E-vector transmission. The filter could be set at the required angle within the last quarter turn of the screw.

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