Wednesday 4 November 2015

Birding Image Quality Tool - Rev. 2.0 Birds and Light

When I initiated this blog in early 2014 I started with the concept of an Image Quality Tool for birders to use to quantify the overall quality of bird images based on key image capture quality parameters including resolution, focus, exposure, colour and artefacts.  Soon after I began to explore the first of my in-depth analyses, namely on the subject of Birds and Light.    The journey has been rewarding and I think it's time to gather the main findings and put them to some use.  One way I have chosen to do this is to augment the image quality tool with another quantitative tool devoted to scene lighting quality.

Once again I have chosen a series if parameters intended to provide a good overall representation of the subject.  Here I will go into some detail on each of these parameters and direct readers to some relevant postings.

Light Quality
There are a number of things that determine the quality of light, perhaps the most important of these being the sun's position in the sky.  As we rapidly head towards winter solstice the light here is already starting to decline in quality.  As I type it is roughly 1.00pm on a very unusually balmy Irish November day.  The sun is as high in the sky as it will get and yet there is a very noticeable yellow tint to the light.  By this hour in mid-summer the sun would be much higher and the light would be harsh, crisp and strongly white.  From here until early spring there won't be too many days in the field when lighting quality will be quite as reliable as it was mid-summer.

I really enjoyed constructing this animated gif.  It represents the changing quality of light during the year.  Imagine you are lying on your back with your head pointing north and your feet south, watching the sun trail across the sky for the day.  Well, in Ireland this animation depicts what you would witness over a 24 hour period. The long days of summer bring a constancy to the quality of light that we quickly take for granted.  In mid-winter we must make do with an ever-changing pallet from a reddish dawn to a yellowish morning and evening light.  Because the sun is low in the sky the light must travel through denser atmosphere to reach us.  The atmosphere scatters the shorter blue wavelengths of sunlight (Rayleigh scattering), with the result that the light is richer in the longer yellow and red wavelengths.  Hence the changing colour of sunlight and the blue colour of the sky.

Light quality can effect bird images in a number of ways.  The most obvious impact is on colour.  Human's have the in-built capacity to adapt to changing light colour temperature by correcting colours to maintain colour constancy.  We call this colour balance or white balance correction.  Cameras are equipped with white balance correction but accurate white balance can sometimes be hard to achieve.  So this is a quality parameter we need to watch for.  In the image quality tool I have captured white balance under the colour parameter and in the lighting tool it makes another appearance here under the concept of lighting quality.  For more on white balance see HERE.

The other key impacts on images are the angle of sunlight and the intensity of light.  I have tended to elaborate more upon these aspects in my various postings on lighting environments.  For instance here Against The Sky the low angle of the sun in the morning can totally alter the appearance of a bird in the sky when compared to later in the day.

Whereas, On Snow And Ice the major issue to watch for is albedo or surface reflectance.  So light intensity is the big factor to consider in that particular lighting environment.

Overall, when we start to analyse lighting quality we begin to establish the optimum lighting quality conditions for birding and photography.  Hopefully through my analysis I have arrived at a good categorisation and weighting system for lighting quality in bird images.  For more on this subject see HERE.

Light Direction
Having analysed the overall quality of the light we are dealing with its now time to take a look at the direction of light.  As observers and photographers we are all acutely aware of the benefit of putting the sun to our back so that our subject is uniformly lit from the front.  Unfortunately we can't always control this.  When faced with a tricky bird identification the lighting conditions are often a very significant factor.  We might feel that judging the sun's direction should be fairly easy in a photograph but very often it is not.  In the posting on Lighting and Shadow Direction I looked at a couple of methods to help pinpoint lighting direction with a finer degree of accuracy.

If the sun is shining and we have positioned ourselves fairly well in relation to the sun and our subject then we may find that the sun's position can be pinpointed due to the specular highlight in the bird's eye.  We may not have an exact three-dimensional direction but we can at least draw a conclusion within the two dimensions of our digital image.

Failing that, if we are lucky we might be able to establish light direction based upon surface normals.  The principal, Lambert's Cosine Law is that light reflection is at it's brightest where the light source hits a surface at a 90 degree angle (or 'normal' to the surface).  Here by drawing surface normals at the brightest points on the surface perimeter of this Ring-billed Gull (Larus delawarensis) I have been roughly able to establish the direction to the light source.  The analysis is consistent with the eyeball method above.  Were the bird to have been lit from the side or rear, so that it's eye were in shade, this technique would offer a workaround to establish lighting direction.

So why is lighting direction of relevance to us?  Well, if we know the lighting direction we can establish the direction in which shadows are falling.  Then when it comes to an analysis of field marks we are in a better position to analyse for potential anomalies caused by shadow.

From the perspective of the image lighting quality tool I have kept this analysis fairly simple for now.  It should be possible to establish if a bird is front lit, back lit or side lit without having to resort to such a fine level of analysis.  But I have presented these finer tools here should such a more critical analysis be justified.

Shadows, as just stated regularly cause confusion during bird identification from photographs.  In fact shadows fall so consistently within the topographical recognisable areas of the plumage that I think an argument can be made for a Shadow Topography.  And in fact, that is just what I have proposed in the posting of that name.

What if the day is overcast I hear you ask?  Well as it happens I have spent a great deal of time teasing out this and related questions, the most recent of which was the posting entitled Lighting and Perspective (Part 2).  Through experimentation I have been able to demonstrate how effective cloud cover is as a diffuser of light.  The simple answer to the question of light direction on a cloudy day is light is scattered in all directions.  Therefore from the perspective of the camera the subject is being lit from the front at all times.  Meanwhile shadows are also being cast in every direction.  The shadows are also soft and diffuse because they are being diluted in effect by the scattered light.  This is why, in terms of observation and photography a bright overcast day will tend to trump a bright sunny day every time.

Dynamic Range
This is one of the technical aspects of photography that can sometimes be difficult to get ones head around.  Put simply it is the range of light intensity that can be captured by an imaging system from the brightest point to the darkest point.  A good way to start thinking about this subject is to consider how well ones eyes adapt when suddenly faced with bright sunlight or total darkness.  Our eyes have an incredibly broad dynamic range, much broader in fact than any digital camera.  And yet we have our limits.  We have evolved methods to adapt to changing lighting, some of which take time to kick in.  In a way a digital stills camera is at a bit of a disadvantage as a still image is not dynamic.  The camera has a brief moment to capture an image.  After that the camera can do no more with the lighting presented to it.

That said there are techniques available that can boost the dynamic range of the camera and indeed simple ways of adapting to the available light, just as our eyes can.  The simplest of these is of course exposure compensation.  By adjusting exposure time, aperture and/or ISO settings a photographer can peer into the brightness or the gloom and even see beyond the dynamic range of the human eye.  But the camera can only shift exposure beyond the human range.  The camera's overall dynamic range has not improved.    For instance, a camera may be capable of capturing the detail on the surface of the sun and also capable of peering into the dimmest corners of the universe.  But it cant do both at the same time.  If it could it would have a dynamic range far beyond that of the human visual system.

Below I have attempted to combine an understanding of camera exposure and dynamic range in one graphic.

There are also techniques that allow the dynamic range of the camera to be boosted.  These are referred to as High Dynamic Range Imaging techniques or HDRI.  I have explored some of these techniques in a posting HERE.
Obviously what makes dynamic range of relevance to us is that, much like the challenging light that makes it difficult for our eyes to see properly, if a camera's dynamic range is exceeded image content suffers and the challenge for bird identification is made much greater.  So how do we establish if an image has suffered due to dynamic range issues?  The answer can be found by studying the image histogram (see image above).  A histogram is simply a graphical representation of all the tonal levels in an image.  If clipping has occurred the graph will show a spike at one or both edges of the histogram.  On an exceptionally bright day, such as the early winter's day when this European Robin (Erithacus rubecula) was photographed the harsh light exceeds the dynamic range of the camera.  In this case the brighter levels (the highlights) have come close to clipping while there is also a considerable accumulation at the dark end of the histogram (the shadows).  In the original image on the left the middle of the histogram is quite low and flat and this reflects the high contrast of the image.   HDRI techniques can be remarkably good at restoring a balance to an image, reducing the overall contrast and in doing so bringing out detail from the mid tones.  The right hand image was created using a HDRI tool.

So, in the context of our overall lighting quality tool we are looking for evidence of high dynamic range issues.  Is the image high in contrast?  Is there evidence of loss of details in the mid tones and is there evidence of clipping?

Multiple Lighting
Without having given much thought to the consequences of Rayleigh scattering or cloud cover light diffusion one might assume that there is only one light source in the heavens.  But when we take a much closer look it turns out that an image is often made up of lighting of different sources and colours.  Take for instance a normal sunny summer's day.  In the sun the light colour temperature will be very close to perfectly white.  But in the shade the light temperature is very different.  The blue sky canopy scatters light into the shadows on a sunny day and renders the shadows blue in colour.  This also has a baring on the appearance of our subject.  I have studied the lighting qualities of different lighting environments in great detail in the posting Lighting Under The Microscope.

This dual lighting can be very frustrating.  There is nothing worse than being presented with a perfect portrait opportunity, like this mega rare Eastern Olivaceous Warbler (Iduna pallida) only to be frustrated by the lighting.  There are effectively two different white balance settings to choose from in this one image, the white balance for the shade and the white balance for the sunlit area.  This image also illustrates the dynamic range challenges created by bright sunlight.  There is some clipping and blooming (an artefact associated with highlights clipping) both evident around the top of the tail and rear toe and claw of the left leg.

Summary and Conclusions
By gathering together various lighting aspects and presenting them here in the form of a concise image quality measurement tool, hopefully I have met my objective of summarising and drawing a line under the chapter Birds and Light.  That said I have found myself repeatedly coming back to this subject because I find it so interesting.  No doubt I will add more postings to this page but I'd like to think I have gathered enough information to provide this broad summary.

No doubt the measurement scales can do with some fine tuning but I am happy for now at least with what I have achieved here.  For those who wish to download and play around with both the Image Capture Quality and Lighting Quality Tools please follow the link below.  Feedback as always welcome and much appreciated.

DOWNLOAD Birding Image Quality Tool Rev. 2.0
(Note you will have to download the file and open in MS Excel for the tool to work properly).

Here is an example of both the Image Capture Quality Tool and Lighting Quality tool in use using one of my favourite images ever - a displaying and aptly named Sunbittern (Eurygypa helias) from Venezuela.

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