Tokina 12-24 f:4 AT-X Pro

I recently bought this lens brand new for my Nikon D90.

The images with my first copy were very soft so I returned the lens and now I have a new copy which is much better, but I have the feeling that it is still not as sharp as it should be.

I have uploaded 2 of the sharpest and 2 of the softest images I got straight out of the camera:
http://www.dev.acreer.com/tokinatest/

would really welcome any comments regarding the image quality, especially sharpness.
My two main questions are:
1 - are the first two images sharp enough?

2 - is the softness of the third and fourth images acceptable for this lens with those settings?

My other two lenses are a 50mm 1.8 nikon and an 85mm 1.8 nikon.

I didn't expect the Tokina to produce as sharp images as the nikons, but the difference is clearly visible.

Thank you for any help and advice.