I have been using my Tamron 28-75 on my D700 and I would rate my copy of this lens as good, but not fantastic with respect to sharpness. I don't know how much variance there is in IQ between copies of this lens.
I must qualify my statement on sharpness by saying that I haven't tried fine tuning the autofocus to see if that helps.
I also bought a Tamron 28-75 for my D700 and I've been very disappointed with it : I could not get any sharpness in the center even with manual focus and there was a lot of coma on both sides. But in this case, I'm sure there was a real problem with the lens because the results were abnormally bad. So I returned it to B&H who refunded without problem. I replaced it with a Nikkor 24-85 2.8-3.5 which gives very good results.
I don't own the 28-75 but it is on my planned purchase list...
How is it after applying USM (or at least in-camera sharpening set to 7 or higher)? I find USM beneficial on most everything I shoot, so that is how I would plan to process images from this lens. Thanks,
SteveK My Nikonians gallery 'A camera is an instrument that teaches people how to see without a camera.' -- Dorothea Lange
Tested my D700 with a Tam 28-75mm and the combo produced very sharp pix at every focal range and at every f-stop. Might be something wrong with your particular sample, or your in-camera settings might need adjusting. Through experimenting, here's what I think are the best settings -- in the shooting menu, under Set Picture Control, set the camera on "SD" (Standard) and then click the "right" arrow on the multi-selector. In the Standard mode, make only 2 adjustments -- set the Saturation option one "tick" to the right of 0, and set the Sharpening option all the way to the right on "9" and then press "OK"
I had issues like this when using 3rd party lenses.
It really is hit or miss with their lenses. I have found that Sigma has been the one that gave me the least amount of issues.
Eventually, I got around to getting all Nikkor glass and the difference is truly day and night.
Third party lenses are definitely a great way to work your way into faster glass at a cheaper price tag so don't get me wrong. You definitely pay A LOT more for the Nikkor glass but you do get what you pay for.