Thursday 20 August 2015

Birds and Light - Foliage Canopy Edge

In the posting Under Foliage Canopy I discussed lighting impacts when studying and photographing birds under a foliage canopy.  Late spring and early summer in temperate forest zones in the Northern Hemisphere is characterised by a vivid green light produced by new foliage growth as illustrated below.  By late summer forests in temperate areas begin to take on some of the characteristics of forests of the tropics.  The vivid green light which baths these forests in early summer gives way to a gloomier light as the canopy closes and as leaves harden and becomes less translucent.  This trend persists into the autumn until deciduous leaves begin to discolour and fall, reopening the canopy once again.  

In this posting I am specifically looking at photography along that boundary, looking into the canopy from outside, where birds move in and out through the foliage.  And, just for clarity, here I am looking solely at mature canopy cover rather than new growth.  So, whereas in these conditions green transmitted light can still certainly play a role, the real lighting challenge on the canopy edge tends to be changing light intensity and not so much the colour of the light.  On a dull day such as today in Ireland it can be very striking just how dark and forbidding foliage canopies can appear.  Due to the dense growth of late summer the light intensity seems to start falling off strongly just inside the boundary of the outermost leaves and branches.  A lot of birding actually takes place on the edge of the foliage canopy, whether it is along a forest path, or along a hedgerow or in a park or garden.  Obviously many birds like to stay close to or within cover so we are frequently dealing with this challenging, changing light.

Metering and Exposure Control
I have covered camera exposure and light metering in depth HERE.  Metering subjects that are moving along and through the edge of a canopy can be extremely challenging due to the nature of the metering process and the fact that the lighting on the subject may be constantly changing.  Sometimes it pays to select a wide metering scope such as Evaluative Metering based on the canopy and subject rather than Spot Metering on the subject alone in order to keep the exposure range narrow, then use exposure bracketing to generate fixed exposure increments in the hopes of capturing a few decent shots.  The alternative is to use Spot Metering and try to carefully track the subject closely and have the camera adjust exposure accordingly as the lighting changes.  But if we are not metering off suitable surfaces we end up with poor exposures as described below.

HDRI and Tone Mapped Images
With bracketed exposures we have the potential to merge exposures to generate High Dynamic Range Images (HDRI).  I have explored HDRI HERE and as the posting demonstrates HDRI is only really practical if the subject is completely motionless and a tripod is used.  HDRI type images can also be produced manually from a single RAW image file, referred to as Tone Mapping.  This may be a slightly more practical solution.  This is also discussed under the HDRI posting and demonstrated with the European Robin (Erithacus rubecula) image below.

Ghostly Subjects
I am currently trying to get to grips with a phenomenon that has perplexed me for years.  Very often when I have observed and photographed a very pale, ghostly subject such as a pale warbler on the edge of a foliage canopy I have been very disappointed with how seemingly unrepresentative the images of it have been.  Quite often the images appear too dark to represent the subject, suggesting that the images may have been underexposed.  Then if I happen to have captured images that look more representative of the bird in life, the images to me often appear overexposed and not quite accurate.  I am sometimes left wondering if in fact these birds are not quite as pale in life as they appear to be and if it could all simply be a Brightness Illusion as discussed HERE.

Here is another example of a very pale bird which for me never quite photographed correctly.  This Marsh Warbler Acrocephalus palustris proved to be a difficult subject to photograph as it only appeared briefly from within the brambles from time to time.  Like the Aberrant Willow Warbler Phylloscopus trochilus example above it was digiscoped from some distance away, which only added to the difficulty of trying to obtain a proper exposure.

As I discussed elsewhere under the posting Grey Scales and Gulls, without having a frame of reference such as a grey card to guide a correct exposure the actual correction for the individual tonal range of a subject can be a matter of pure guesswork.

Exposure and RAW Tone Mapping Experiments
Using a lux meter I carried out a simple experiment to test how light drops off as one penetrates the foliage canopy.  I selected a single example of three common trees and started with a reading out in the open and a number of readings at arbitrary distances underneath the canopy but without disturbing the leaves.  This simple experiment reveals just how quickly and effectively a mature canopy absorbs the available light.  In the case of the Leylandii Cyprus 99.9% of the light was absorbed within a half a meter of the canopy edge.  Both the Alder, Alnus glutinosa and the Common Hawthorn, Crataegus monogyna absorbed 80% of the light before I reached a point where I couldn't proceed any further without disturbing the canopy.

Next, to demonstrate how slight changes in exposure and distance from the foliage canopy edge can alter the appearance of our subject I placed a number of identical white targets on the surface of leaves of an Alder, from the very edge of the canopy to approx. 20cm and 40cm inside the edge respectively.  The day was overcast and dull when all these images were taken.  A drop of a few hundred lux in this case within a few centimetres of the canopy edge clearly had a fairly dramatic impact on our subject.

I took a bracketed exposure involving normal, 2 stops over and 2 stops underexposed.  By shooting in RAW we can maximise the quality of our images as discussed HERE.  I selected the best of the three bracketed exposures and opened up the RAW version of the image.  I then set about Tone Mapping the image by eliminating clipping at both ends of the histogram and trying to bring out the maximum tonal range on all three targets.  The results show a significant improvement on the earlier results.  This may be the best way to obtain consistent results when photographing birds on the foliage canopy edge.  The challenge with all photography is to obtain an image that represents the tonal range of our subject.  Bracketed exposures, coupled with tone mapping gives us a good chance of achieving that goal. 

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