Tuesday, 24 March 2015

Birds and Light - Under Foliage Canopy

Light and shade in the forest
For many of us in the temperate zone in the Northern Hemisphere the spring equinox marks the first real signs of regrowth within the temperate forests.  Lighting under a foliage canopy is challenging at the best of times but the chlorophyll-rich, soft, new growth of the late spring and early summer presents an especially tough obstacle for observers and photographers alike.

Green Transmitted Light
This green light which bathes northern deciduous forests in early summer is put to use by many a dingy warbler during nest building and while gathering food for a hungry brood.  For the observer, this light can increase the challenge for bird identification.  This is one of the reasons most birders would probably consider forest birding the most challenging of all environments. An autumn, winter or spring tristis-type Chiffchaff is somewhat less challenging to photograph and white balance correctly owing to the relative lack of foliage.

Possible Siberian Chiffchaff Phylloscopus collybita tristis, Ireland in spring carefully colour balanced using a combination of grey card and DNG profile (more HERE).  However, faced with the same bird under a foliage canopy it is far more difficult to appreciate and photograph it's subtle buff tones.

I have already looked at this challenging light in detail HERE.

Inca Jay Cyanocorax yncas, Venezuela.  Note that, while white balance correction using a grey card does a remarkably good job, it may not be possible to remove all of the green cast, especially around the edges of our subject, where nearby foliage may increase it's local intensity.  A white balance correction is uniform across all pixels so it relies on the colour cast being completely uniform.  Which, of course, it rarely is.

Key to this diffuse light is the translucent property of leaves.  As leaves mature they become more opaque and the canopy also closes, absorbing most of the remaining light, so very little light reaches the forest floor.  This green cast is diminished and is replaced by a darker, gloomier light as the summer progresses. These are also the conditions one might associate more with tropical and evergreen forest birding.

Low Light Intensity
In rainforests there are specialists for every niche, including many that inhabit the gloomy undergrowth and floor.

Plain-backed Antpitta Grallaria haplonota, Venezuela, photographed under a very dark rainforest canopy.  Luckily it remained still and I was able to control camera shake for the 1/50th second exposure by propping the camera lens against a log.  The green haze in this instance is defocused foliage between the camera and the bird.  Without a grey card and no neutral greys in the shot, white balance in this case was by trial and error. The same goes for the image below.

Andean cock-of-the-rock Rupicola peruvianus, Venezuela.  The exposure time required for this image was 1/6th of a second.  Without a tripod it took quite a few attempts to obtain a sharp image.

Dappled Light and Other Factors
The light transmittance by leaves is also governed by the intensity of the daylight above plus the usual impacts of shadow, cloud cover and time of day.

If forest birding and photography wasn't challenging enough, perhaps the most difficult task of all is to obtain an image of the bird in the open, unaffected by shadows and obstacles.

Dappled light plays havoc with exposure and dynamic range and, in the process introduces artefacts.

To conclude, photographs taken under a foliage canopy will often present with image quality issues.  The lighting in this environment is challenging for various reasons and it can take a lot of patience to extract an identification from these kinds of images in many cases.

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