Friday, 24 April 2015

Field Marks - False Contrast

Contrast is not recorded by the camera sensor.  It is one of the corrections given as part of the process of creating an image from a RAW data file.  Together with saturation, focus and white balance, the degree of contrast correction applied is typically set by the camera, with limited input by the photographer.    Most modern digital cameras make a reasonable stab at contrast adjustment when automatically generating JPEG images, but for various reasons, photographers may like to further adjust contrast, often together with the overall brightness of the image.  Or, contrast may be inadvertently changed due to other forms of post-processing of images.  The inter-relationship between contrast, saturation, brightness and focus correction during post-processing is surprising, as explored HERE.  Alternatively, and preferably of course, contrast can be added while working directly from RAW.

Contrast may be naturally low, for example on a foggy or overcast day.  Or, it may be naturally high, for example on a bright sunny day or, exacerbated by a bright reflective environment such as snow, ice or a white sandy beach.  Cameras cannot replicate the dynamic range of human vision, so bright days give rise to unnaturally high contrast images, with a significant loss of tonal range.

Faced with all these challenges, image contrast is not an easy thing to get right.  More often than not image contrast doesn't paint a totally accurate representation of the subject.

So lets face it, "false contrast" is common place.  Why does contrast matter?  Well, in terms of field marks, contrast can be very important.  For the most part, the preference for appreciating field marks correctly would be a naturally low contrast, high dynamic range image, where all colours and field marks are represented by as wide a tonal range as possible.  For more on high dynamic range imaging (HDRI) see HERE.  In categorizing field marks broadly into the Bold and the Bland, contrast is identified as a key parameter distinguishing the two.  Bold field marks tend to exhibit high contrast relative to features around them, while bland field marks are often low in contrast, blending in with their surrounding features.  Bland features also tend to require a broader tonal range so high contrast is not compatible with the full range of tones required to properly display many bland field marks.

It is also possible to demonstrate through an analysis of field marks that various image quality parameters impact on contrast, which in turn impact on the appearance of field marks.

Of course, the primary focus of this blog is bird identification from digital images.  Poor contrast correction in digital images can have a real baring on bird identification.  Lets take for example the identification of Catharus thrushes here in Europe.  The main confusion species on this side of the water is the Song Thrush (Turdus philomelos).  Song Thrush is noticeably bigger than any of the Catharus species but size can often be difficult to assess in the field, and of course is generally not possible at all from photographs.  One of the useful features separating Song Thrush from all of the Catharus thrushes is the contrasting pattern of the ear-coverts (auriculars).  In Song Thrush there is a noticeable contrast, whereas in the Catharus species, while the pattern may be similar it is more subdued.  If image contrast is poorly corrected it can have a baring on a difference like this.

In the first image below the contrast of the Song Thrush image has been lowered.  More specifically, the mid-tone shadows have been brightened with the result that the contrast of the ear-coverts has been reduced together with the shadows.  Subtle corrections like this can easily go unnoticed.  The contrast in the ear-coverts is less obvious than it might have been in life.  This is one of the problems with HDRI if it is done without consideration for features that are obscured within the shadows.  For more on lighting tools see HERE.

In the image below the contrast in both images has been dramatically increased.  Effects like this can occur naturally due to harsh lighting.  In fact the image of the Song Thrush looks quite natural, and indeed it is.  The image was taken on a cold winter's day.  The lighting was naturally quite contrasting when the image was taken. The image of the Swainson's on the other hand looks a bit off and that is because the high contrast added does not tally with the visual cues in the image.  

It is important to state of course that, while we may think we have a good sense for the lighting in images, humans are actually not well equipped for this type of analysis and we frequently get it wrong.  

In addition to the natural contrast caused by lighting, contrast can be artificially added or removed in an attempt to compensate for poor exposure, or even due to the over-sharpening of images using unsharp mask.  

While a very experienced observer would still manage to identify this tricky pair, even from these gaudy images, someone less experienced, perhaps focused too closely on one or two field marks could easily fall foul of images such as these.  

Whether excessively low or high contrast is due to natural conditions, camera settings or the over-use of post-processing tools, the message is the same.  It pays to take stock of the overall quality of an image including lighting and composition before delving into the finer details.  Field marks can't be looked at in isolation.  It is also important to allow for the possibility that what we perceive from an image may be, at times, a poor representation of what we see in the field, and more importantly, a poor reflection of how the bird would look, up close and in good light.  Excessively low contrast can make bold field marks appear bland.  Excessive high contrast on the other hand can obscure bland field marks completely but at the same time might elevate subtle patterns into false bold field marks.

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