Originally posted by zapp remove the yellow sticker!
f/5.6 is hard for a K5 to autofocus, adding that it is a third party lens, you probably have to go manual.
Next step is to make sure that your camera is aligned for maual focus. Use live view to compare viewfinder and screen sharpness.
Also, 400mm focal lengt with 1.5 crop factor is a VERY LONG focal length. A basic tripod will not support this lens combo - especially not at close inspection. First time users will often produce too much camera shake.
Blame the lens or blame the CAMERA.
Iv just sent the lens back now,the lens I had was manual only but would not manual focus on anything at all, no yellow stickers on the class just a really bad lens with big problems I think am going to save for a new lens now