D800 + Nikon 28-300 ED F3.5-5.6G corner of images darker

welcome to slr photography...you have just discovered vignetting! as you have just found out, it can be a pain and, as such, is one of the standard metrics by which lenses are judged...most lens testing sites measure vignetting. here's your lens on photozone:

http://www.photozone.de/nikon_ff/578-nikkorafs28300vrff?start=1

you'll note quite high numbers wide open at 28mm. looks like it clears up nicely by f/8 so for landscapes, stop down and you should be ok.

in general, all lenses vignette to some degree. it's very common in ultra fast lenses and a lot of high ratio zoom lenses. luckily, it is quite easy to correct in post in an application like lightroom.

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dave