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.

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