Thursday, 20 November 2014

Birds and Light - On Snow & Ice

Light and Shade on Snow & Ice

At Sea birds are simply moving around in an environment consisting of water and air.  In many ways, a snow and ice environment is much the same.  The key difference between observation and photography in a watery environment when compared with snow & ice is albedo, or surface reflection.  Freshly fallen snow, with a surface reflectance of 80 - 90% has the highest albedo of any naturally-occurring environment on the planet.  But this can rapidly decrease as snow melts and absorbs soil and other material.  Water has among the lowest reflectance from above but as we know is highly reflective when viewed at angles increasingly approaching the horizon (i.e. at a high angle of incident light).

A group of Antarctic Terns Sterna vittata resting on drifting glacial ice in a Chilean fjord make for an interesting juxtaposition between ice, which has among the highest albedo values and water which has among the lowest.  But water has an unusual property in that surface water reflectance varies depending on incident light angle.  Glacial ice also has a distinctly blue colour which sets it apart from the white ice and snow on land. Glacial ice is also not as bright as other ice.  It actually differs from sea ice (HERE) and it's blue colour is explained HERE.

The intense reflection from snow and ice makes observation and photography a real challenge, particularly as the sun gains height.  Snow and ice particles scatter white light which can have the effect of illuminating a subject from all directions. This is rather like the fill lights used in a portrait studio.  It can be an advantage under the right conditions.  The problem occurs when subjects are not very uniformly lit and the contrast between very bright patches of light and poorly lit areas is high (high dynamic range).  If dynamic range exceeds the camera's abilities tonal detail will be lost through clipping - especially likely for a pied subject like this Snow Bunting Plectrophenax nivalis.  That said, if shooting in RAW, much of this detail may he recoverable and this type of scenario actually suits a form of image optimisation referred to as ETTR (exposure to the right), as referred to HERE.

However a much more common problem involving snow and ice scenes is underexposure.  This occurs due to incorrect light metering, whereby the camera's selected exposure overcompensates for the brightness of the terrain and the subject is underexposed as a result.  For more on light metering and related exposure issues see HERE.

The upshot of all of this for identification of birds from photos is to be mindful of the lighting conditions, time of day, location and latitude and the image exposure.

Howell and Dunn (Gulls of the Americas, 2007), for those who have access to it, discuss under Environmental Factors (pages 13-18) the difficulties of trying to photograph gulls in this type of environment.  Plate I.19 on page 17 for me is a good example of an image which has been underexposed.  It is a particularly interesting image also because there is a fill light element to the image which, to me appears like it could be due to low sunlight reflecting off an ice wall or bank behind the photographer (as opposed to a camera flash for example).  The overall lighting effect is eerie and almost other-worldly.  The bird's mantle shade appears too dark for Vega Gull Larus [argentatus] vegae but it's bill and legs and the ice around the bird are also oddly dark, and this reveals the actual exposure issues with this image.  This is a good example of the kind of bizarre lighting anomalies that can be associated with snow and ice.  I'd like to thank Amar Ayyash for drawing my attention to this image.

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