<p>To echo all the other posters, if your 14-24 is "not sharp", then it needs service.<br>
At 15mm, you have the Zeiss and the Nikon 14-24. The Samyang is good, but not <em>as</em> good.<br>
At 18mm, the 14-24 is the best 18mm you can buy. The Zeiss 18 is very good, but has way more illumination falloff than the 14-24....<em>way more.</em> The old time Nikon Ais 18's have an image rendering that's very, well,... old-timey. But they could be excellent for smaller prints or web use.<br>
At 21mm, the Zeiss and the 14-24 are so close it's a draw, imo. I'd pick the Zeiss 21<strong> <em>if</em></strong><em> </em>I knew when I leave the house that 21mm was the only focal length I'll need.<br>
The Zeiss 25/2 is very, very good, but I wouldn't discount the Nikon 24/1.4 either. The Zeiss 25/2 has an aperture ring and no AF motor. I like that. </p>