Saturday, 28 February 2015

Field Marks - White Balance Test

In The Bold And The Bland I explored the distinction between two classes of field marks.  Bold field marks are characterised by rich, saturated colours and/or uniform, contrasting and clearly defined markings.  Bland field marks have the opposite characteristics including desaturated and or very bright or dark colours, and/or diffuse, low contrast and ill-defined markings.  A field exposure test confirmed these characteristics (as repeated below).

I have taken these images and applied a cold white balance error to each.  

Thus far with our Exposure Test and Focus Test we have seen a consistent pattern.  Bold field marks are impacted far less by image quality errors than bland field marks.  It can take very little to degrade bland field marks, making them difficult or impossible to judge without some form of forensic analysis.  Here, having applied a significant white balance error we appear to be seeing much the same pattern emerge.  The bold colours, such as the yellow in the Euphonia and in the crown of the Firecrest remain clearly visible but the subtle tones on the breast and back of the Pipit and Wheatear are less clear.  It should be borne in mind however that white balance errors will affect all colours so if we need to sample and record colours as accurately as possible we do need to try and correct any white balance error that might exist.  The purpose of this exercise was merely to illustrate that bold colours are more resilient to colour balance errors than bland colours.

Friday, 27 February 2015

Field Marks - Focus Test

In The Bold And The Bland I explored the distinction between two classes of field marks.  Bold field marks are characterised by rich, saturated colours and/or uniform, contrasting and clearly defined markings.  Bland field marks have the opposite characteristics including desaturated and or very bright or dark colours, and/or diffuse, low contrast and ill-defined markings.  A field exposure test confirmed these characteristics (as repeated below).

I have taken these images and blurred them using Gaussian blur tool to closely mimic natural lens defocus.  That way we can directly compare reasonably focused images with defocused images to see what additional impact focus is having on our field marks. 

What we find is that the largest and boldest of our bold field marks (eg. the white belly of the Dipper and yellow plumage marks of the Euphonia) can still be worked with even in heavily defocused images.  Whereas, bland field marks, and the smaller or finer of our bold field marks (eg. the fringes on the Wheatear's wing) cannot be distinguished at all.  The reason for this can be explained by the anatomy of focus.  Defocus occurs at the pixel level and the amount of defocus is measured as pixel radius.  Each defocused pixel impacts on the pixels around it, out to the edge of the pixel radius.  Defocus has a greater impact on smaller objects in an image because these tend to be impacted upon by a greater range of features, colours and tones around them.  In this case the black and white patterns of the Dipper breast, triangular graphic and Firecrest facial pattern are all big enough and bold enough to withstand this level of defocus while the smaller, narrower feather fringes of the Wheatear are obliterated by this level of defocus.

Pixel radius and defocus are illustrated by the graphic below.  It illustrates the typical characteristics of defocus, including:- that it spreads out in a circle from the point source, that contrast is progressively dissolved along with edge sharpness the further away the blurring travels from source and that it occurs right down to the pixel level.

It is interesting to note that the yellow crown patch in both the Euphonia and Firecrest is almost obscured.  Once again these patches are so small as to be virtually obscured by this level of defocus.  This brings me to another point.  Defocus blends the colours and tones of pixels, often out to a surprising radius from the original pixel.  The best chance of sampling colour accurately from an image containing defocus is to find the centre of a large, uniform patch.  For more on this see HERE.

In summary, what we see here is a consistent pattern.  The boldest field marks remain the least impacted by an image quality parameter.  The smaller and narrower of the bold field marks don't fare as well.  Bland field marks are the least resilient.

Wednesday, 25 February 2015

Field Marks - Field Exposure Test

In The Bold And The Bland I explored the distinction between two classes of field marks.  Bold field marks are characterised by rich, saturated colours and/or uniform, contrasting and clearly defined markings.  Bland field marks have the opposite characteristics including desaturated and or very bright or dark colours, and/or diffuse, low contrast and ill-defined markings.

I have printed a number of images, including a few I recently used in the blog and I have presented them together with my X-Rite Colorchecker and an additional graphic.  Here below is a reasonably neutral photograph of the rig taken in reasonably bright light outdoors.   

Next I created a series of exposures from under to overexposed and I have presented snippets of these images below for comparison purposes.  While this illustration may appear a little cluttered hopefully the results speak for themselves.  The test does confirm what we would expect to see.  A drastic loss of image quality obliterates bland colours and markings while we can still make out bold colours and markings, even in terribly under and overexposed images.  

The bold field marks represented here include the white breast of the Dipper, bold yellow of the Euphonia, sharply defined facial markings of the Firecrest and contrasting feather fringes of the Wheatear.  The bland field marks include the subtle black fringes on the back of the Dipper, the breast shaft-streaking of the Pipit,  the plain mantle of the Wheatear and the subtle breast colouration of the Firecrest.  

The graphic at the centre of the test rig allows for a clearer appreciation of what is actually happening to the various images during under and overexposure.  Underexposed images obviously are darker.  In terms of the tonal range of the image this means that brighter tones are darkened while the darkest tones are clipped.  So there is an overall reduction in tonal range and a loss of contrast.  In trying to appreciate bold field marks we can cope with this loss of tonal range and contrast.  But for bland field marks the loss of tonal range more often than not results in a masking of the colour or detail we are looking for.  For overexposed images we have the opposite taking place.  Darker tones are brightened while the brightest tones are clipped.

It is worth noting that the smaller, narrower of the bold markings are not performing as well as the larger of the bold markings.  For instance the very narrow black lines and marks are very faded in the overexposed images, as if by blooming (an artefact).

It is interesting to note that optimum exposure may not always mean that we have the optimum conditions for analysing an individual field mark.  Some field marks may be expressed more clearly if an image is slightly over or underexposed.  Note for example that the subtle black fringes on the back of the Dipper are more clearly visible in the overexposed image.  This is because the tonal range has been broadened at the black end of the tonal scale.  In the posting HERE I outlined how Lighting Tools can be used to bring out hidden details in images.  The same basic principals apply.

See also HERE.

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.

Saturday, 7 February 2015

Field Marks - A Summary of Field Marks

To borrow from gaming and virtual reality parlance, field identification is an immersive experience.  Experienced birders use all of their senses plus their experience when out in the field.  Identification of birds from photographs is a far more limited sensory experience.  But what it may lack from a sensory perspective can be more than made up for using extra-sensory skills - namely the forensic tools that help us get more from our images.  So, photo identification is also an immersive experience, but a rather different one than that to which we are accustomed.  When we compare these two 'realms' we find that certain aspects of field identification and photo identification are common to both.  The majority of my postings over the last few months as it happens have fallen within this area of overlap including of course my recent focus on identification through field marks.

I started looking at field marks first from the perspective of typical avian anatomy, feather tracts and the anatomical contours of a bird.  This developed into a discussion about lighting and how light and shadow normally interacts with the anatomy of a typical bird.  Afterall a photograph is merely a light map.  To develop that analogy further, if we understand topography and can visualize contours and land marks we can read a topographical map.  Avian topographical features might be considered akin to topographical features on a map.  The contours of a bird are like the contours on a map.   Field marks are the land marks we are looking for to help us figure out where we are.  If we misinterpret what we see we will be led astray.  By extension, if a map is bland and featureless and we have no landmarks to guide us we cannot find our way.

Having discussed avian anatomy I took a look at avian bare parts, again with a particular focus on lighting.  The translucency of bareparts (and indeed plumage) introduces an interesting additional dimension and potential source of confusion.  I then looked at a range of typical plumage field marks working from the centre of feathers outward.  I started with shaft-streaks and tramlines,  Next I considered the body of the feather in feather centres including subterminal markings.  Lastly to feather fringes, notches and tips.  The final posting in the series touched on colour field marks.

So, what are the conclusions from this brief overview of field marks?  Well, firstly it is clear that the environmental lighting factors which confound field identifications also largely impact on identifications based on photographs.  Lighting and shadows can play tricks on the observer.  When faced with an individual photograph, lighting can really throw the unwary.  But, with a bit of care and experience it is possible to read the scene lighting and take account of it when analysing image content.  It is hard to best a good quality photograph when it comes to critical, forensic analysis.  A single image captured in a moment can reveal more information about a bird than hours if not years of field observation.

Having reviewed different types of plumage field marks what probably stands out most is that it is the tonal complexity of any given field mark that possibly matters most.  Image exposure and dynamic range are key to the capture of any field mark in an image.  It helps if the field mark is quite uniform in tone.  If there are a gradient of tones there is increased risk that the marking will be at least partially obscured in an image.  The worst conditions for capturing many field marks are bright sunny days with broad dynamic range.  The best conditions for capturing all field marks but most especially subtle markings and colours are in bright but diffuse (i.e. overcast) light.

It is clear that the correct ageing, pattern of moult and wear and even feather alignment can all play a key role in many identifications.  Being able to analyse this information relies obviously on a good understanding of the criteria and field marks first and foremost.  It also relies on being able to patiently study and read an image correctly.  Last, but not least, the image needs to be of good enough quality to allow for such a careful and critical analysis.  In other words, there are no short cuts!

Field Marks - Colours

Colour is complex.  I have already covered it in some detail in earlier postings.  I would encourage a read of the summary HERE and also to consider colour in the context of other field marks HERE, particularly giving consideration to lighting, anatomy, translucency and wear.

Field Marks - Fringes, Notches and Tips

Contrasting fringes, notches and tips are a common feature of many species, most typically on the feathers of the wings and tail.  These can form important field marks.  Typically they are a feature of fresh plumage and they wear and vanish over time.  Here is a reasonably fresh juvenile Wood Sandpiper (Tringa glareola) with neat white notching displayed throughout its upperparts.

Below we have two juvenile Semipalmated Sandpipers (Calidris pusilla) photographed in Ireland in September.  The top bird is a classic, fresh bird while the lower bird is heavily worn or abraded.  Presumably the lower bird hatched much earlier in the summer, but even at that it's plumage is in very poor condition for this time of year.  Note how the while fringes of the mantle are almost completely worn away.  The abrasion of the tips of the lower scapulars splits the feather ends causing diffusion of the remaining fringe, so it is far less obvious.  Note the scapulars are cloaked over more of the coverts in the lower bird but the coverts are also heavily abraded and misaligned - almost clumped together and not neatly fanned like the coverts on the bird above.  Interestingly this worn bird also displayed aberrant behaviour, tending to skulk in dense grass much of the time, while the upper bird was content feeding on the open beach with other waders.

Northern Wheatear (Oenanthe oenanthe) is an incredible migrant.  Populations ranging across the entire northern hemisphere migrate over vast distances, including open ocean to winter in Africa.  The fresh juvenile/1st winter bird below is already starting to lose it's buff-coloured fringes through wear.  Initially these are being worn and/or bleached whiter leaving a barely visible subterminal band of buff, which will also fade.  When this bird returns to Ireland in spring most of it's summer plumage will consist of the very same feathers but these fringes will have completely worn away.  Note feather abrasion and bleaching may introduce very fine pale fringes to otherwise uniform-looking feathers.  These are wear patterns as distinct from pigmentation patterns.

Red-breasted Flycatcher (Ficedula parva) is separated from the fairly recently split Taiga Flycatcher (Ficedula albicilla) through a combination of subtle features including the darkness of its uppertail coverts relative to it's tail (lighter than the tail in RBF and darker in TF) and also by the subtle pattern of the spot at the end of its tertials (thorn-shaped in RBF and round in TF).  The subtle buff tones to the fringes and the breast are also relevant.  As feathers fade and wear RBF will naturally more closely approach TF in appearance.

Thanks to feather arrangements, pale fringes naturally contribute to larger patterns and features such as a pale secondary panel (eg. the Northern Wheatear above), but such features are dependent on the bird maintaining a certain posture and therefore need to be interpreted with care.  Take for example this juvenile Red-backed Shrike (Lanius collurio).  Quite a number of species exhibit this subtle secondary panel.  The presence or absence of this feature depends on individual variation but also as we see it depends on feather wear and feather alignment.

As if things were not complicated enough we also have to consider certain artefacts, most notably edge halos, produced due to image sharpening algorithms, and, one of my favourites, moiré.

Most  moiré shouldn't be too much of a problem as it tends to occur perpendicular to the direction of predicted plumage fringe patterns. However as shown by the example above, extremely low resolution can give rise to a cross-hatch moiré pattern.  This has the potential to mirror the impression of scalloping patterns.  I will be looking at this and other false field marks in future postings.

Finally, here is a rare example of a bird showing dark feather fringes (White-throated) Dipper (Cinclus cinclus).  Cormorants (Phalacrocoracidae) are another group of birds that display this field mark.  I wonder if there is a link between this subtle type of dark scalloping and the underwater environment?  And, if so what is the advantage for these species?

Monday, 2 February 2015

Field Marks - Feather Centres & Subterminal Marks

Feathers can carry a great array of plumage patterns.  Diagnostic patterns form field marks which are used for identification purposes in many species.

Subtle Patterning
The centres of the feathers of many birds are solid or fairly uniform in colour and tone, free of any patterns.  And, that is how they are generally depicted in field guides  However, lighting can give the impression of false plumage patterning or tonal variation.  Because feathers do not sit flat on the surface of a bird there will invariably be a gradient created by the varying angle of parts of the feather relative to the observer/camera and the light source.  Take the Little Stint (Calidris minuta) and Semipalmated Sandpiper (Calidris pusilla) comparison below.  The exposed feather centres of the upper wing, including the scapulars and tertials of juvenile Little Stints are typically a fairly uniform blackish.  In juvenile Semipalmated Sandpiper the upperwing feathers show a very different pattern, consisting of a gradient from pale feather bases towards darker tips, combined with a dark shaft streak.  The effect is like a diffuse, dark arrowhead or anchor shape near the feather tip.  This is considered a very useful difference between Little Stint and similar species, but in this particular shot, due to the sun's angle, there is an impression created of a very similar diffuse gradient of tones from the base to the tip of each feather in this juvenile Little Stint.  This impression is due entirely to lighting and only occurred momentarily from the observer's position.  In the field, as the bird moved about, the true pattern of these feathers would have been clearer.  But, such is the nature of a single photograph - the bird, it's lighting and composition are all frozen in time.  The lighting is similar in the Semipalmated Sandpiper shot but the angle is slightly better.  The primaries give a useful clue to the very different lighting going on in these two images.

Clearly these two birds are at the more typical ends of their respective plumage spectra.  So, there are plenty of additional clues to their true identities.  But, if one were faced with an exceptionally grey or worn Little Stint or a more richly-coloured Semipalmated Sandpiper the challenge posed here might have been more difficult.

Identification of Stints and Peeps by the late Peter J. Grant and Lars Jonsson (British Birds: 293-315, July 1984 - HERE) is a classic.  Peter J. Grant was a pioneer of The New Approach to Identification and Lars Jonsson's plates are simply breathtaking.   It is interesting to compare Jonsson's plates (eg. 113 and 114) with the image above and to note Jonsson's very subtle yet perfectly accurate use of lighting.  The problem of course is that such optimal, neutral lighting conditions, which are so typical of most illustrated bird identification plates, are not often encountered during field observation, and are even more challenging to capture as accurate digital images.  And yet, there is possibly no other way to meaningfully treat these birds on a single plate - I find the results to be mesmerising each and every time I revisit this paper.  Arguably no collection of photographs could have done this subject more justice.

So, it is hard enough to cover a complex identification puzzle without having to consider lighting and other photographic challenges as well.  Of course Grant and Jonsson were well aware of the various challenges with photography and referred briefly to these in their paper (page 300).  This whole blog in many ways is elaborating on the concepts briefly discussed on that page.

The image above roughly illustrates the impact of lighting and exposure on subtle plumage markings. Below I have a worn juvenile Semipalmated Sandpiper photographed in dull light.  The tonal range is good and the contrast low, allowing for a much richer appreciation of the subtle tones in each feather than in the image shown above, however colours are possibly not as saturated as they might have been in brighter sunlight.

Bold Patterning
Detail is much more likely to be captured and interpreted correctly in a photograph when plumage markings display natural contrast.  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.

While it might seem like a contradiction in terms, some bold patterning is actually subtle in other ways.  Take the Spotted Sandpiper (Actitis macularius) below.  It combines pale and dark subterminal bars on its upperparts, enhancing the visibility of plumage markings as a result.  But it appears as though these markings are intended to be bold at close range but less so at a distance.  The subtlety lies in the choice of colouration.  Still, these more sharply-defined plumage features are certainly easier to interpret than for example the arrowhead markings shown by Semipalmated Sandpiper above.

However, there is also a flip-side. Digital image sharpening tools and the human visual system use a similar method to enhance contrast and acutance (the apparent sharpness of edges).  This means that if the image is low in resolution it may be hard to distinguish between this form of plumage pattern and similar-looking digital artefacts.  Beyond a certain range, all markings, however bold cannot be resolved.

Markings like these are relevant in some identifications.  In Spotted Sandpiper and its European relation the Common Sandpiper (Actitis hypoleucos) the exact pattern of these markings on the tertials and coverts help clinch an identification.

Here is another example of a bird that utilises bold markings but in a subtle way.  This is a Rufescent Tiger Heron (Tigrisoma lineatum) from Venezuela.  At very close range its bold camouflage markings are apparent but at range they are scarcely visible.

Cryptic Plumage
While on the subject of subtle camouflage I might as well briefly look at the question of cryptic plumage.  There are probably no hard and fast rules for the accurate capture of plumage field marks when it comes to these birds because the sky is the limit as regards markings, colours and tones,

Many cryptic species are nocturnal.  For photographing nocturnal species, obviously light availability is the biggest challenge.  The Great Potoo and Paraque below were both photographed under relatively low torch light.  The birds stayed motionless for long periods, allowing for longer exposure times and a close, careful approach using a vehicle as a hide.  Long exposures can be deceptive - any subtle movement (eg. feathers moving in the breeze) can create motion blur which can mask plumage features.  This blurring can easily go unnoticed especially if the exposure is particularly long.  Shadows can also be deceptive in these artificially lit images.  For shyer, more active species including Owls getting useful images can be very difficult.  Photography under artificial light requires careful white balance correction.

But of course not all cryptic species are nocturnal.  Nocturnal species presumably developed cryptic plumage mainly to avoid being predated upon by diurnal predators as they slept.  Not a bad tactic even when you are awake!  There are plenty of diurnal species with complex cryptic plumage as well, eg. Eurasian Wryneck and Sunbittern shown here.  It was a revelation to see just how well camouflaged this Sunbittern remained even while actively displaying.

For many species with cryptic plumage there are few if any confusion species to be concerned with so accurate capture of cryptic plumage markings may not be a consideration.   But for many (eg. Snipes), identification does often rely on obtaining high quality photographs.  The subtleties of image quality parameters and forensic image analysis don't probably come into the discussion in that instance.  Poor quality images simply will not cut it - enough said.

Sunbittern (Eurypyga helias), Venezuela