Front Focussing of Sigma 24mm/1.8, Nikon 85mm/1.8 and Tamron 17-50mm/2.8

Surprise, Surprise! Got my Sigma 24mm back (remember the quick comparison with a Tamron 17-50 a few weeks ago). A professional photographer bought it and returned it a few weeks later, because of focussing issues. He suspected that it might suffer from Front Focussing. This is a problem with some third party lenses, where the autofocus focusses a point in front of the object and not exactly the object plane.  A short explanation and a quick test can be found at Photo.net. A more thorough test procedure is here. So I tested the Sigma and the two other lenses I own with the quick prodedure on my Nikon D80. This procedure also reveals tendencies of chromatic aberations in the defocus areas.

Results
The Nikon 85mm/1.8 focusses perfectly fine, but shows strong vertical chromatic aberrations in the defocus areas: Green behind the focussing plane and magenta in front. The Tamron 17-50mm/2.8 focusses well, with – maybe – a slight tendency towards back focussing. There is nearly no chromatic aberration. The Sigma shows a clear front focussing affect and mild chromatic aberations.

Have a look at the test images:

Nikon Nikkor 85mm/1.8
_DSC4423.JPG

Sigma 24mm/1.8 EX DG Macro
_DSC4421.JPG

Tamron 17-50mm/2.8
_DSC4417.JPG

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