ISO 100 is very low when shooting handheld with a long lens. Just a thought: If your other lens is giving you better results, it might be that it's because it's a shorter focal length, and just under the usable handheld shutter speed threshold. I usually stick to "safe" shutter speeds of at least 1/whatever the focal length I'm shooting at. For what it's worth, I've never had a problem with my 55-300, ever. It's a superb performer and one of my all-time favorite lenses.
Originally posted by Noskovus I don't really use autofocus. I was just mentioning that it was even worse when I tried it.
What AF mode are you using? If you're using a multi-pattern mode, it might be that it's focusing on something other than your subject. Use center point AF for awhile & see how that works. Otherwise, you might try adjusting the autofocus, That could also be your problem.
If you want to verify whether your focus is working correctly, stick a piece of newsprint to a well-lit flat wall, put your camera on a tripod several feet away, directly facing it (not at an angle), use center point AF, open up the lens all the way, and take some photos at various focal lengths. If you have a remote it would be a good idea to use it to eliminate the possibility of camera shake. If the pics are in focus, the lens is fine. If not, you need to adjust the AF.
You could have a bad copy of the lens, but I'd try to rule out other things first.
Good luck,
Bob :-)