Originally posted by gazonk The main difference is that the DA15 shows more CA, but isn't that correctable?
Yes it is correctable - however
nothing can be done to correct for the lacking corner sharpness of the DA15mm f/4 ASPH when compared to the Sigma 8-16 even at f/8 on a Pentax K5IIs.
the sigma lens has higher contrast in the branches against the sky (approx 500:1 contrast) the DA15 has
slightly better contrast in the tree leaves (50:1 contrast) the Sigma has slightly reduced contrast at these but it can resolve higher frequency detail. Bear in mind the DA15 is stopped down two stops from its widest aperture, and the Sigma zoom has only been stopped down one stop from its widest aperture @15mm - this is pretty remarkable performance from a zoom lens. Most SLR lenses perform at their best at two stops down from their widest apertures.
Pentax K5IIs - Sigma 8-16mm f/4.5-5.6 @ 15mm f/8 uncorrected - Vs Pentax DA15mm f/4 ED ASPH @f/8 - corrected in Lightroom - no sharpening.