Saturday 11 October 2014

Birds and Light - At Sea

Light and Shade At Sea

Because the ocean is very uniform, there are few obstructions, reflecting or transmitting light on a subject. All we have are the sun, the sky and the sea.  There are basically just a handful of lighting conditions to deal with at sea.

The time of day, the angle of the bird and camera relative to the sun and whether the day is overcast or sunny all determine how plumage colours and patterns are presented in digital images.

There is a predominance of blue light at sea, together with white light.  After having spent a few days at sea I have found myself craving greens and reds, as they have been lacking at sea!  The image below explains the predominance of blue light in the atmosphere and at sea.

Colours At Sea

As the image above discusses, the colour of the sea can vary depending on its depth and the constituents dissolved and floating in it.  But there tends to be a reasonable uniformity over a relatively small area of sea.

I photographed this Great Shearwater Ardenna gravis from the bow of a tall ship as it passed directly beneath.  This inky dark water is the natural colour of the deep ocean.

The other great influences on the sea are the sky and the prevailing weather conditions.  Overall, the colour pallet of the backdrop (sea and sky-scape) tends to be fairly limited and very blue in colour as explained above.  On a sunny day this can be an advantage because the colour pallet of most seabirds is at variance with this, typically blue pallet.  

The image above represents the colour temperature scale, which, put simply, is the typical variation in the colour of sunlight on earth between dawn and dusk (for more see HERE).  At sea, this scale closely matches the colour pallet of light, of the sky, and in many cases, the colour of the sea itself.  After all the sea is a big reflector as explained above.  In contrast to this, the plumage of most seabirds consists of earthy browns and greys, with the odd dash of brighter colour (normally in the bareparts).  Seabirds also tend to have a lot of pure white in their plumage and this can be handy as white also tends to mirror the colour of light reflecting off of it,

In the posting Colour By Numbers I outlined how colour in digital images is expressed.  The colour of each pixel is defined based in three parameters.  Luminosity is the brightness of a pixel and is directly related to the light intensity of that point in the scene being photographed, but is also influenced by camera exposure and dynamic range (see HERE).  Hue is the actual colour - it corresponds closely with the colour of a specific wavelength of light, but in reality is the net result of multiple wavelengths coming together at that one point.  Lastly, Saturation is the purity of the colour and is expressed as a percentage of grey mixed with hue.  The parameter that most interests us here is hue.  If we can take a look at the hue of the pixels making up the bird we should be able to determine what are likely to be real plumage features, as opposed to false markings produced by shadow and reflection on white plumage.  Note that shadows can either have a blue or grey cast to them at sea.  On a sunny day, shadows are blue because of the predominance of blue in the light coming from the sky.  Sky light is the light that illuminates the shadows.  It is also the light that reflects off the sea and illuminates the underside of the bird.  When the sky is overcast the scattered light peering into the shade is actually diffuse white light, therefore white areas appear grey in the shade, on an overcast day.

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