Sunday, 15 June 2014

Birds and Light - Lighting under the microscope

Grey Card Comparisons

I have been playing around with a test rig consisting of an X-rite Colorchecker and some additional targets mounted on a white board.  HERE I took a closer look at White Balance correction and explained a bit more about the rig.  I first put the rig to the test in a verdant forest setting and was really amazed by the results as outlined HERE.

Below I compare a variety of commonly encountered lighting settings.  I will continue to add to these in due course.  Here is a summary of what I have been uncovering.

A Tale of Two White Balances

Firstly an important point.  It is possible to have more than one correct white balance in any given image.  When there is more than one source of illumination in a scene, each illuminating a different part of the scene more or less independently, then it is indeed possible to correct for each of these light sources independently.

Surely the sun is the only source of illumination in an outdoor scene, one might ask?  While the sun may be the ultimate source, different entities within the environment, which reflect or transmit "modified" sunlight, themselves act effectively as alternative light sources.  Examples include the sky which transmits a powerful blue light, foliage which can transmit an intense greenish light and water which can reflect a virtual mirror image of an environment onto a subject, including indeed a mirror of the subject itself, in all its various colours.  Obviously, in order to correct for different sources the grey card must be placed in the path of each source individually.  The x-rite colourchecker allows for the simultaneous correction of two different light sources in the same scene as outlined below.

GOLD - Optimum Lighting 

The optimum light for observation and photography is a bright and overcast day.  This is due mainly to the light scattering affect of moisture droplets that go to form clouds.  Unlike the scattering created by gas molecules in the atmosphere which mainly just scatters the lower wavelength blue portion of daylight, moisture droplets scatter light of all wavelengths equally well.  The result is a soft, diffuse, neutral light, which baths a subject quite uniformly, even scattering into deep shadows. 

SILVER - Harsh Daylight 

There are a couple of issues with bright sunlight.  Cameras are limited in their ability to simultaneously deal with high luminance bright sunlight and the low luminance shadows left in its wake.  The ability of a camera to deal with light of variable luminosity is referred to as it's Dynamic Range.  For most digital cameras the result of taking a photograph in bright sunshine is a high contrast image with some burnt out details and colours.  Parts of the image are generally underexposed and parts are generally overexposed.

There is also an environmental affect.  The high intensity sun heats up the molecules in the air causing them to bounce around resulting in heat haze, which affects image focus.

From a white-balance perspective we also have, in effect, dual illumination going on as shown in the "tale of two white balances" example above.  The sun is casting a highly polarised, direct white spotlight onto a subject, while, at the same time, the blue sky canopy is casting scattered blue light on the subject from various directions.  As this blue light is much dimmer than the sun it is overpowered by the white light on that portion of the subject illuminated directly by the sun's rays.  However, in the shadows, where the sun's direct rays cannot penetrate, the blue light from the sky creates a clear blue colour cast.  

A single grey card cannot simultaneously deal with dual sources of illumination, unless of course the card happens to be positioned at the boundary between two independent light sources (e.g. partly in sun and partly in shade, as the example above illustrates).  Either way however, we can only make one correction to an image at a time.  Either we live with the blue shadows or we correct for those and end up with a yellow colour cast in the mid-tones and highlights.  Harsh daylight really is not ideal, either for observation or photography.  When trying to judge colours accurately the key point to remember is to verify highlights are properly white balanced and remember then that shadows will contain a bluish colour cast.

BRONZE - Deep Shadow

Deep shadows tend to be associated with bright sunny days.  They represent a gloomy juxtaposition to harsh, bright daylight.  As the above test proves, deep shadows are naturally bluish in colour due to the influence of blue sky canopy light.  However if the whole subject and grey card are both in shade there is a chance that a single grey card white balance correction will eliminate this blue cast.  The results below show that, while there is a dramatic improvement, there does appear to be a residual blue cast even after white balance correction.  The other interesting finding is the tendency for targets to show a much more obvious reflection, or projection of the bright background, i.e. the sources of light peering into the shadow.  The overall result therefore suggests that shadows are not a uniform blue cast as they might appear, but are rather more akin to a cinema projection of the surrounding environment.  While it may not appear as dramatic as this (it appears as a subtle dappled affect) I think the presence of complex and bright background colours might impact our ability to judge colours accurately in the shade, even after a good white balance correction.  It would seem therefore that an image of a bird in direct sun offers a better chance of judging colours correctly than an image of a bird that is in deep shade.



Foliage Canopy
Though this particular foliage canopy test possibly out-performed either the bright sunlight or the deep shade tests, typically birds are not too cooperative in these environments, so shots tend to involve dappled light or tend to be under-exposed due to extreme shade.  A useful test nonetheless, particularly the revelation that reflected and transmitted light from foliage can be quite uniform in nature and can be more or less eliminated with a grey card white balance correction.

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