Fri 05-Apr-13 02:24 AM | edited Fri 05-Apr-13 02:27 AM by agitater
I see evidence of camera shake, possibly slight downward movement of the camera as the shutter is pressed, and subject movement. There may also be some lens softness at play here too.
The medium shot of the bird on a branch is simply misfocused. The lens was all the way out at 500mm, the image seems to have been cropped (if I understand your previous posts correctly), so the original subject must have been quite small in the viewfinder. There seems to be enough contrast in the subject for a good focus lock, but at the distance I think you were shooting it's probably difficult to get a good focus lock. It seems like the camera locked on the branch rather than the bird. If the subject bird was under a non-cross type AF point, the camera was probably struggling a bit.
There's also just a hint of haze in the shot. Shooting at 500mm through a bit of haze in the air makes for soft looking shots. I think you should consider trying to get much closer to your subject matter.
The Sigma 150-500 is a bit of poor performer at 500mm at f/11. Mind you, center performance at f/11 is about as good as that particular lens can do, although it's not rated very well at that length. The lens has been reviewed a number of times, and most of the reviewers complain of literally hazy looking shots at 500mm. The lens is a winner from 200-400mm, by all accounts, from f/5.6-f/11.
Get much, much closer to your subjects, straighten your horizon, keep your focal length between 200-400mm, increase your shutter speed (something much faster than 1/500th at those focal lengths - at least 1/800s), keep your aperture at f/8 or larger, and I think you'll be much happier with your results.