Wednesday 2 September 2015

Birds and Light - Against The Sky (Part One)

In this series of posts I have been exploring the various different lighting environments in which we observe and photograph birds.  In many ways watching and photographing birds against the sky is the ultimate, pure synthesis of birds and light.  

It's hard to resist a dramatic sunset scene.  Take this party of Black-bellied Whistling Ducks (Dendrocygna autumnalis) coming to a Venezuelan evening roost.  These silhouetted birds may not be the easiest to identify from this image but this scene is actually a very good representation of how my eyes witnessed this spectacle.  Very often however when we photograph birds against the sky the results don't really match expectation or indeed what our eyes are capable of seeing.

Dynamic Range, Metering and Exposure
The posting on High Dynamic Range Imaging (HDRI) discusses the camera's particular limitations when it comes to dynamic range.  For those not already familiar with this concept I recommend starting there.  I spent a bit of time filming a Chimney Swift as it endlessly circled a patch of scrub at the edge of Baltimore village, Co. Cork one morning in late October.  For light metering and exposure I was completely at the mercy of the Sony mini dv camcorder I was using, and it was a very frustrating exercise!  Typically all digital cameras respond to the brightness of the sky by reducing exposure to preserve highlights.  This has the inevitable effect of underexposing everything else in a scene.  But when this bird flew low against a darker background the image brightened up and the bird's colour tones and detail were revealed.

I am very fortunate where I live that in almost any weather throughout the year my neighbour's racing pigeons circle my house in small parties, and it makes for a nice spectacle.  It also affords me the opportunity to practise my flight photography and to try and make sense of this lighting environment.

As this sequence of images illustrates, even in continuous shooting mode the camera is constantly influenced by the overall scene lighting.  Exposure is adjusted accordingly with each frame.  This sudden shift in exposure can be even more dramatic along the interface between the sky and the land or built environment, as illustrated below.

There is a limit to what can be done to resolve this particular problem.  We could attempt to narrow the focus by spot-metering but where our subject is fast moving it is simply impossible to track the bird with such a high degree of accuracy.  Alternatively we can dial in exposure compensation or use exposure bracketing. There are pros and cons to all these methods.  If we fix an exposure compensation but then a bird all of a sudden offers us the ideal photo opportunity our fixed exposure compensation may result in a missed opportunity.  Exposure bracketing on the other hand increases our scope a bit but also increases the amount of rubbish shots we will end up having to sort and bin.  In the end it's probably a matter of personal taste, trial and error.

Lighting Variation
In most of this series of postings I have tried to simulate how a subject looks under various different lighting conditions from pre-dawn to sunrise and throughout the day to sunny versus cloudy conditions.

Taking this presumed female Venezuelan White-tipped Swift (Aeronautes montivagus) for example we can see that lighting angle, white balance and light intensity all create very different effects.  The sky is always brighter than the ground and our subject.  This is particularly striking before dawn and after dusk when subjects appear strongly silhouetted against the sky.  Little or no detail is apparent in these conditions.  At sun rise and sun set the low angle of the sun illuminates the underside of a high flying bird.  Plumage detail can be revealed that is not normally visible and this can easily confuse an observer.  Take for instance the very similar Common Apus apus and Pallid Swifts Apus pallidus.  The yellow light of early morning and late evening can dramatically alter the colour and appearance of a juvenile Common Swift on migration, leading to potential mis-identification as a vagrant Pallid.  For more on this challenge see part two of this thread HERE.

As the sun gains height in the sky typically the underside of the bird falls into shadow and the outer wing and tail feathers start to become more translucent looking.  A bird takes on a much different appearance.

I have discussed translucency in detail HERE.  Of course depending on the angle of observation relative to the sun it is possible with patience to obtain good views and photographs of both the upperparts and underparts of a flying bird but, for the most part our views tend to be somewhat limited by lighting.

In bright overcast conditions we may benefit from slightly better overall lighting and viewing conditions but birds still tend to remain effectively in shade or silhouette much of the time and this just gets worse as cloud thickens.  Overall, there is no ideal or optimum viewing conditions for watching birds against the sky.  Each set of circumstances carries its advantages and its challenges.

Processing from RAW and other formats
As various earlier postings have illustrated there is a huge advantage to shooting RAW.  Images which have been over or underexposed can be rescued.  While working from JPEG or a similar format there is far less scope of resolving exposure problems.  Taking for instance a frame from the Chimney Swift video and trying to correct for underexposure doesn't greatly improve the results and only brings noise and other artefacts to the fore.

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