Sun 09-Sep-12 04:11 PM | edited Sun 09-Sep-12 04:39 PM by MotoMannequin
This has nothing to do with the DX designation.
You may have noticed that your lenses will appear to zoom very slightly as you adjust focus, even if you haven't touched the zoom setting. This is called "focus breathing" and some lenses do it a lot more than others.
You should find that your 70-300 & 18-300 give a very similar view at 300mm when focused at infinity. As you focus closer, the 18-300 will "breathe" more than the 70-300.
To your question of whether the 28-300 will be better than the 18-300? I don't specifically know the answer to this question, but my instincts tell me that the focus breathing is a necessary consequence of designing a super-zoom so I wouldn't expect either to do as well as your 70-300.
Edit: To take this a bit further, if you don't mind getting technical...
I looked up the max reproduction ratios of these lenses: 70-300 = 1:4 28-300 = 1:3.1 18-300 = 1:3.2
Which means, as close as you can focus, the 70-300 can fill the frame with something 4x the width of the frame, or on DX 4x24mm = subject 96mm or 4" wide. The 28-300 and 18-300 should will both be able to magnify the subject about the same, in this case they can fill the frame with something about 3x24mm = 72mm or 3" wide. Both these lenses offer slightly more magnification than the 70-300!
How is this possible? Let's look at the minimum focus distance: 70-300 = 150cm 28-300 = 50cm 18-300 = 45cm
Here you have it. You can move the lens much closer to your subject with either of the superzooms, and this gets you more magnification.
To look at this another way, you get get about 33% greater magnification with the 28-300 or 18-300. You might think that at the same focal length, you'd achieve this by moving 33% closer, but because of focus breathing you need to get 300% closer to get this additional magnification!
If your subject is skittish (like hummingbirds) then having a closer min focus distance might not be an advantage.
Anyway, by these numbers, the 28-300 offers very slightly more magnification than the 18-300, and does so at a very slightly longer distance, which tells me that the 28-300 doesn't "focus breathe" quite as much as the 18-300. But these are both "heavy breathers" and for practical purposes I'd consider their performance in this respect to be identical. You'll probably be disappointed if you exchange lenses in an effort to correct this issue.
Again, this has nothing to do with DX vs. FX. The DX designation just means the lens has been built to a smaller diameter to cover the smaller DX frame.