Is the 18-105 soft for distant subjects?

Leonard Shepherd wrote:

I think you are saying most of your pictures are sharp, but a few, including this one, and not.

If I have understood you correctly the likely cause of the problem is the detail on the individual sky supports used as a autofocus target is too small for auto focus to accurately detect.

As a guide with a D7000 the size of the autofocus detail should be at least half the size of the viewfinder autofocus mark. The nearest support, just below where the three legs join, would probably have been a better autofocus target, but it is not in the middle of the picture area.

Nikon give some guidance on difficult autofocus subjects on page 93 of the camera instruction book but the guidance does not deal well with this type of autofocus problem.

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Leonard Shepherd
Many problems turn out to be a lack of intimate knowledge of complex modern camera equipment.