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.

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