Perhaps I will be showing my extreme ignorance here, but here goes anyway! I shoot with a D7000 and have been very happy with the results from my 70-300 FX, especially at 300. Thought I might like a bit broader range though, so I just got an 18-300 DX. I did this accidentally- I thought I was ordering the 28-300 FX.
Decided to do a side by side comparison of the two lenses, so I went to my deck and set up, as I commonly do, to do some closeup shots of the Hummingbirds that frequent my feeders. Shot with the 18-300 at first, zoomed to 300. Thought something didn't look quite right- the birds did not seem to be as "close" as usual. After a few minutes, I went into the house and changed to my 70-300. Went back to the same spot, left the manual settings the same, zoomed to 300, and looked through the viewfinder. Now the hummingbirds were filling up the entire viewfinder, and on review, the images from my 18-300 were definitely not as close a zoom. Looked more like shots taken with my 70-300 at 200 mm.
Is this because it is a DX lens? If so, would the 28-300 be the same as my 70-300 at the 300 end? And is it a good lens for the D7000? I've seen some mixed thoughts on that. Regardless, the 18-300 is going back.