Tuesday 17 February 2015

Field Marks - The Bold And The Bland

Some online sources define field marks very broadly, including anything we might use in the field to identify a bird.  Some might consider for example a bird sound or the distinctive gestalt of a species to be all part of the field marks lexicon.  I prefer a more literal definition for field marks.  I chose here to define a field mark as any distinct marking or colouration which is visible in the field.  By this definition all field marks have the potential to be recordable on a digital image.  Note I am going to leave structural features out of this discussion for now and just focus on colours and patterns in birds.

From the analysis so far I think we are starting to see a pattern emerge.  On the one hand we have bold field marks, including bold colours.  These tend to be quite 'resilient' during most viewing and photographic conditions and are generally recognisable in all but the poorest quality images. Then we have bland field marks.  These include subtle colours and markings which tend to be difficult to capture accurately in all but the highest quality images.

Bold Field Marks
The boldest colours, which stand out in the field, have a high colour saturation and a relatively high luminance.  However colours in nature do not translate exactly into digital colours in images.  Even having deployed the best colour management techniques, a colour which is extremely vibrant in life may appear relatively more subdued in a digital image.  This may be due to a number of factors not least the colour gamut of the camera and screen (i.e. their rather limited colour and tonal pallet; usually 8-bit, sRGB).  Dynamic range (the range between the lightest and darkest tones) is often narrower in a digital image than in life and similarly we have a restricted tonal range to work with.  Despite all of this, the colours in an optimally exposed and properly colour balanced image can often appear very close to reality.  One need only compare the colours in the live image on a screen of a modern smart phone with the scene it is capturing to appreciate just how sophisticated and accurate digital imagery has become in a few short years.

In digital images the colours which stand out best tend to be similarly those with the highest colour saturation (i.e. approaching Sat 240) but tend to be those closest to mid-range luminance (i.e. Lum 120).  If I were to define a bold colour in a digital image I might therefore refer to it as having say a saturation falling within the range Sat 160 - 240 and a luminance within the range Lum 80 - 160.

Take the Thick-billed Euphonia from Venezuela below.  It's yellow plumage is particularly vibrant and this is particularly reflected in the saturation value.  Colour saturation is in fact maxed out at Sat 240 or 100% saturation.  The blues and violets of its upperparts are somewhat more subdued and this is reflected in the saturation and luminance values.  When this bird is in deep shade or when an image is very underexposed the yellow will still retain some colour saturation whereas the blue's will be harder to discern and may in fact appear to be black. This is even apparent from within the shadows of this well exposed image.

The sharp among you may have noticed a flaw in the argument with regards to this particular example.  The blue and violet colouration in this case is more likely due to Structural Colour than pigmentation.  The odd thing about structural colour is it is only visible from a certain angle (eg. iridescence) so there may be more going on in the shadows of this image than meets the eye.  When considering the impacts of light on colours in a scene one has to factor in the mechanism behind the colour.  For more on structural colour see HERE.

Bold markings like bold colours tend to stand out also.  They too could be thought of as having certain consistent traits.  They tend to contrast strongly with surrounding features, tend to be quite uniform in tone and tend to show sharp, crisp edges.  The white markings on this male Moussier's Redstart from Morocco make it easy to locate and identify in almost any setting.  This is a desert bird that wants to be noticed.  The female of the species on the other hand is well camouflaged.

Bland Field Marks
Bland field marks including bland colours have characteristics that oppose those of bold field marks and colours.  Bland colours may display a low colour saturation (approaching Sat 0).  They may range in luminance but tend to be easier to gauge when they have a mid-range luminance (i.e. Lum approx. 120).  Bland markings may contrast poorly with surrounding features.  They may be far less uniform (i.e. consisting of a wider tonal and/or colour range).  Finally, they may exhibit blurry or diffuse edges, making them harder to see clearly.  Many desert species are especially bland.  This Trumpeter Finch from Morocco typifies the subtle plumage colours and patterns found in arid and semiarid areas.  Only the bill of this male approaches anything resembling a bold field mark.  This is perhaps not too surprising when one considers the subtlety of the colour pallet of the terrain it inhabits and the relative lack of sheltered places to hide from predators.

Juvenile Eurasian Turtle Dove (Streptopelia decoacto) starts out with diffuse-centred scapulars and coverts, moulting to contrastingly dark-centred, broadly rufous-fringed 1st-winter feathers.  The moult contrast is very obvious late in the autumn.

Light and Field Marks
Light is such a fundamental part of photography that it raises its head in nearly every posting in this blog.  I have referred to it in all the various postings so far in this series covering field marks.  In Lighting and Avian Anatomy I outlined how light and shade often interact predictably with the typical contours of a bird.  In Lighting and Bareparts I drew particular attention to the properties of translucent features, particularly how colours can be affected by light passing through bareparts.  Lastly, taking each of the typical plumage field marks in turn, I looked at the impacts of lighting and exposure on various markings.  When we take these findings and apply them to the analysis of bold versus bland field marks we can draw some useful conclusions I think.  Strong light obliterates subtle field marks.  It can also influence the accurate capture of bold field marks but they are rarely lost entirely.  Diffuse light is optimal for capturing all field marks both bold and bland, but accurate exposure is also essential.

Image Quality and Field Marks
When we consider each of the image quality parameters in turn we start to see another consistent pattern emerge. This pattern is also seen in the Gaussian-like nature of image quality parameters as discussed HERE,  The pattern to which I am referring is the tendency of image details and colours to be degraded then obliterated as quality deteriorates.

The characteristics of bold field marks means that they are more resilient to image quality deterioration while bland markings are more susceptible.  Lets look at these characteristics more closely.

Defocus and, most especially both under and overexposure impact on luminance.  Once again, on balance bold colours are more resilient to the shift in image brightness.

Contrast and Tonal Range 
All image quality parameters affect image contrast and tonal range.  Bland field marks may contrast poorly with surrounding features so any further loss of contrast may obliterate these features entirely.  It might seem counter-intuitive but, if you think about it, bold features are often quite uniform in tone, whereas bland features can often display a broader tonal range.  It is the combination of tonal uniformity and the contrast of this tone with the tones of surrounding features that make bold features stand out.  Consider for instance how clear and effective written text appears on this screen.  Yet this clear message is delivered using just two contrasting tones, one for the text and the other for the background.  If we increase tonal range eg. by introducing a pattern or gradient to the background, the text becomes less clear.  Reduced quality may result in a compression or clipping of tonal levels.  Generally speaking bold features are once again far more 'resilient' to the effects of reduced contrast and tonal range brought about by deteriorated image quality.

Edge Definition or Sharpness
Though not an essential feature of bold field marks, more often than not, where we see a bold colour or marking its edges are fairly clearly delineated.  Once again, with reduced quality we tend to see a blurring of edges.  This may be due to real image blur (eg. defocus or low resolution pixellation) or it may be simply an impression of unsharp edges caused by lower acutance (eg. low contrast and tonal range).  As with the other characteristics, what we see is a tendency for bold field marks to remain easier to distinguish and interpret than bland field marks, even when defocused.

Colour Saturation - An Afterthought
When I initially looked at this parameter I took for granted what my eyes and what logic might dictate.  It is reasonable to assume that if contrast decreases due to under or overexposure, colour saturation should do the same right?  In fact colour saturation works independently of exposure as explained HERE and HERE.  Every day learning something new.

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