I don't think it's chromatic aberration. The "purple defringing" slider
under lens corrections in ACR significantly reduces the saturation of
the purple fringe there but does not eliminate it.
There are two types of chromatic aberration, axial and transverse and
this image doesn't seem to fit either. Axial CA appears throughout the
image while transverse CA doesn't show in the center but gets gradually
worse the greater the angle off-axis. The only place we see "chroma" in
this image is a small area nearly in the center. Therefore this is not
chroma but "purple fringing". The following is from the Wiki article on
chromatic aberration, the photography section that discusses purple
fringing:
<http://en.wikipedia.org/wiki/Chromatic_aberration#Photography>
-----------------------------------------------------------------------
The term "purple fringing" is commonly used in photography, although not
all purple fringing can be attributed to chromatic aberration. Similar
colored fringing around highlights may also be caused by lens flare.
Colored fringing around highlights or dark regions may be due to the
receptors for different colors having differing dynamic range or
sensitivity – therefore preserving detail in one or two color channels,
while "blowing out" or failing to register, in the other channel or
channels. On digital cameras, the particular demosaicing algorithm is
likely to affect the apparent degree of this problem. Another cause of
this fringing is chromatic aberration in the very small microlenses used
to collect more light for each CCD pixel; since these lenses are tuned
to correctly focus green light, the incorrect focusing of red and blue
results in purple fringing around highlights. This is a uniform problem
across the frame, and is more of a problem in CCDs with a very small
pixel pitch such as those used in compact cameras. Some cameras, such as
the Panasonic Lumix series and newer Nikon and Sony DSLRs, feature a
processing step specifically designed to remove it.
On photographs taken using a digital camera, very small highlights may
frequently appear to have chromatic aberration where in fact the effect
is because the highlight image is too small to stimulate all three color
pixels, and so is recorded with an incorrect color. This may not occur
with all types of digital camera sensor. Again, the de-mosaicing
algorithm may affect the apparent degree of the problem.
------------------------------------------------------------------------
Note especially the discussion about about purple fringing around
highlights and especially about blown highlights. The area of fringing
follows much, but not all, of the contour of his light colored cap which
is also in bright sun. This area does not now measure as blown but may
have been before you started processing or perhaps was only blown in the
green channel. Also, looking at his right hand there is a great deal of
motion blur. I suspect that he also moved his head slightly and the
purple fringe area is somehow mixed up with motion blur and blown
highlights. In any case I don't think this is any fault of your lens.
Fixing this is going to take some PS work. I think I'd try a selection
on the fairly uniform purple color followed by desaturation or color
change to match the cap or any of several other possible methods to
cover it up or eliminate it.
Chuck Norcutt
On 2/22/2014 1:39 PM, Tina Manley wrote:
> The Remove Chromatic Aberration button in LR did not work on this one:
>
> http://www.pbase.com/tinamanley/image/154578993
>
> It has done a remarkable job on most of the awful CA from the Canon wides,
> but it barely touched the purple fringe on his cap. UGH!
>
> Tina
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