AF-ON on D600 ... hit and miss. Me or the camera?

Could be a lot of things casusing the difference. Hard to tell exactly as the EXIF data is missing. If you upload a jpeg directly as it comes from the camera to your gallery here (no cropping or PP in another program)
Then go here http://www.dpreview.com/members/settings/profile
And under the "allow download of originals"  check  "Allow visitors to download full size original images from your gallery" That way we can download the pic to ViewNX2 and check the camera settings you used. That assumes you uploaded pics/files as they came from the camera vs any change in Photoshop or the like.  In this case PP may have erased the EXIF so a simply conversion to Jpeg from the RAW file using ViewNX2 will solve that. Be sure and install the latest version from the Nikon web site and not the version that comes with the camera. Good Luck

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My opinions are my own and not those of DPR or its administration. They carry no 'special' value (except to me and Lacie of course)