Originally posted by johnyates The DA 15 is really sharp . . . in the center of the frame. However, it exhibits a pronounced curvature of field. If you like your landscape pictures sharp across the field you're better off with one of the zooms discussed above.
A good point however if you stop the DA15 down to f/11 and use hyperfocal focusing the field curvature isn't such a problem. The Sigma UWA zoom lenses typically require thin 82mm* Polariser filters and these are
considerably more expensive than the regular 49mm filters you can use on the DA15.
*The Sigma 10-20mm f/3.5 uses 82mm filters, the slower Sigma 10-20mm f/4-5.6 EX uses 77mm filters - either way they need slim polariser frames to avoid vignetting at the 10mm setting. Also the flare tolerance of the DA15mm f/4 is nothing short of remarkable, Sigma lenses on the other hand
do have issues with flare, especially the 8-16mm f/4.5-5.6.