Here be Unicorns


Photographer Colin Prior, who took this photograph says: I am fascinated by the fleeting nature of the optical phenomena I observe outdoors, such as rainbows, the Aurora borealis (northern lights) or an eclipse.

During my time in wild places, I have been privileged to witness some great rarities — including a white rainbow, the Earth Shadow, a Brockenspectre, the green flash of a setting sun and the Fata Morgana (a mirage) in the Arctic Circle. Each of these phenomena presents their own
unique photographic challenges and for a photographer, require an
understanding of the factors, which create them.

Rainbows are coloured arcs with a radius of 42 degrees around the antisolar point that appear directly opposite the sun. A fainter secondary arc can occur at 51 degrees and when both primary and secondary are present we have a ‘double rainbow.’ Viewed from an aircraft, a rainbow can inscribe a complete circle not visible at ground level. Rainbows can be found in
unexpected places such as waterfalls or geysers. At the beach we can see ‘surf bows’ and there is a ‘marine bow’ formed from the prow of a ship. Whether it is formed in the spray from your garden hose held at arms length, or in a sheet of rain a few miles away, the angular size of a bow remains the same.

Photographing rainbows present a number of problems; primarily the need to use an ultra wide-angle lens if an entire bow is to be included. On a 35mm camera, a lens of 20mm or wider will ensure full coverage of the arc above the anti-solar point. Where the bow is at some distance, the use of a telephoto lens can be used to give more emphasis, with the inherent compression of the lens design adding value. A polarising filter is extremely useful and increases saturation by eliminating most of the spectral reflection from the droplets of water.


In the image of Glencanisp in Sutherland, squally showers were blowing in on a north-westerly air stream across this ancient landscape – the weathered towers of Canisp, Suilven, and Stac Pollaidh are composed of Lewisian Gneiss some 300 million years old and are surrounded by a sea of Torridonian sandstone, somewhat younger at 100 million years old. It was an early April evening and the sun had begun to set. As this heavy squall began to pass I
recognised the potential for a rainbow to form and in anticipation, set up my tripod as it still poured. Within minutes the sun had broken through and was illuminating the tail end of the shower forming both and primary and secondary arcs – the primary one being the most vibrantly coloured. I managed to photograph four 6×17 frames on my panoramic camera before it faded as quickly as it had appeared.

Of the two white rainbows I have witnessed, one in the Cairngorms near Drumochter, the other near Loch Merkland in Sutherland, neither photographed successfully. When temperatures are below freezing, the spherical water droplets are replaced by ice crystals which are more complicated shapes and fail to refract into a true spectrum and form a white bow which is just
about as rare as Unicorns!

One evening in February, I found myself on the summit of Stob Ghabhar near Inveroran, specifically to photograph the snow covered peaks of Glencoe.
Frustratingly, the sun was setting further north than I had anticipated and
I turned my attention in the other direction towards Ben Dorain and the
Crianlarich Group. As twilight approached, the drama intensified and I shot
a series of images as the shadows began to engulf the mountains – such was
the success of the shoot that I chose one of the images for the front cover
of Scotland – The Wild Places.

With the sun gone, the temperature began to plummet and my thoughts were
solely focussed on descending, however, I remembered that a full moon was
forecast that evening and I lingered on the summit in the gloaming to see
where it might rise. Within fifteen minutes a purple sphere was rising over
the Rannoch Moor, through the earth shadow and into the antitwilight arch.
It was a phenomenal scene and I shot a dozen frames on the Fuji GX617
panoramic fitted with a 180mm lens.

The earth shadow is best seen from a high elevation (such as a mountaintop
or aircraft) when the sky is clear and there is a long line of sight. For a shadow to be visible it must be cast upon something and in this case it is cast upon the atmosphere itself. As the sun sets, the boundary between the earth shadow and the antitwilight arc rises in the sky and becomes progressively less distinct.

What our eye-brain perceives during a twilight event is difficult to record photographically due to ambient light levels. In some instances when photographing the twilight arch or earth shadow with a wide angle lens, the natural fall off of light inherent in the lens design may result in images
where the centre is correctly exposed but the edges of the frame are considerably darkened. With transparency films, bracketing is the best way to ensure the optimum result and those photographers working with digital cameras should consider RAW capture, to give as much flexibility to replicating the image which was directly observed at that moment.

For walkers, climbers and photographers, naturally occurring optical phenomena offers us the opportunity of witnessing events that we have never seen. It also should encourage us to ‘see’ what we may have been looking at for many years in a state of blindness and to believe that ‘here there be Unicorns!’

Colin Prior November 17th 2004

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