Thanks for the file Paulo. The problem is common to most internal focusing fast tele lenses including the Sigma 150mm f2.8 macro and 50-150mm f2.8 zoom. You will notice that colour fringes in front of the focus plane, and behind the focus plane, appear magenta and green respectively. What is happening is that the floating element system used for internal focusing is maintaining near apo-chromatic correction in the focus plane, at the expense of increasing longitudinal chromatic aberration in front of and behind the focus plane.
This is a complex issue, but it means that if you took the lens and focused it using its own focus mechanism (internal, or with floating groups) then mounted it on a bellows which could reposition it to focus on a different distance, it would be appear to be a very badly corrected lens with strong longitudinal error. In normal use this error only affects defocused zones and it takes special conditions - such as extreme backlight on an edge - to reveal it.
The artefact which appears in images is a tendency for slightly out of focus detail in front of the focus plane to be more correctly focused by long wavelengths (red light) while shorter (blue) are more diffused and contributed less to the visible core image sharpness. Behind the focus plane, blue light is more accurately focused. In the digital image, the result is magenta tinting of blurred detail close to the camera and green tinting of detail further away. I've been through all this with the Sigma 50-150mm f2.8 which managed to show black lettering on an angled test target as pink and green either side of black focused detail for one of our writers, who was very confused and thought it must be a faulty lens.
This effect is absent in lenses such as the Schneider Apo Symmar or Rodenstock Apo Sironar. It is present wherever the lens design is not symmetrical and fully apochromatic. It only makes itself clearly visible with very fast, longer lenses on the digital medium.
You have processed the image in such a way that this is exaggerated. It is possible to process the image, at full size resolution, to obtain a result which looks as clean as it would on film - where the same effect happens, but you are not able to boost it to view an extreme result.
Here's a full size version of the small file (now on your own space):
In fact the image when processed neutrally to cope with the extreme contrast is pretty good, with an attractive tiny diaphram star of light caught on part of the flower which is in sharpest focus. My processing settings were:
Tone Curve Linear
Space sRGB
Sharpness 25
Colour Noise 50 (this helps reduce chromatic edge effects)
WB as shot 4750 +28
Exposure -2.00
Shadows 0
Brightness +25
Contrast 0
Saturation 0
David
(click on the thumbnail to see the larger image)
