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The Nikon 28-300mm ƒ/3.5-5.6G produced some odd results for sharpness, which may or may not be objectionable. As the lens is intended for a full-frame audience, I will begin with our observations with the lens mounted on the D3x.
Perhaps the most striking result we obtained was with the lens set to 28mm and ƒ/3.5. We've started coining the term ''sombrero'' to describe this performance - softness in the center, sharpening up out towards the corners, and the some more corner softness. Fortunately, stopping down even to ƒ/4 dramatically reduces this phenomenon, and by ƒ/5.6 we see performance we would expect from a thousand-dollar lens - a sweet spot of sharpness in the center, degrading to slight corner softness. Stopping down at 28mm doesn't produce further sharpness, until ƒ/16 where diffraction limiting removes the central sharpness. This performance is also observed at the 35mm setting.
In the mid-range (50-105mm) we don't see the sombrero effect, just some soft corners, when used wide open. In these focal lengths, ƒ/5.6 produces a reasonably sharp center degrading to soft corners, and stopping down to ƒ/8 produces slightly more overall sharpness.
At the telephoto end (200-300mm) the lens produces mediocre results when used wide open (ƒ/5.6), but fortunately, stopping down just one stop to ƒ/8 cleans it up - the image is overall just slightly soft, with corners that are just slightly softer. Stopping down further than ƒ/8 doesn't produce any sharper results.
Fully stopped-down results are nothing to write home about - while the images produced are consistent from corner to corner, they're soft at the wide end (28mm, ƒ/22) and downright blurry at the telephoto end (300mm, ƒ/36).
The above commentary is essentially applicable to the sub-frame images produced on the D300s, with the caveat that soft corners are less of a factor given the reduced field of view available in that camera.
So the question is, especially at wide end showing the sombrero pattern, will the average user notice this effect? As a complement to our sample images, we shot a series of photographs of a flat brick wall, with both the Nikon 28-300mm, and a lens we know to be incredibly sharp, the Sigma 70mm ƒ/2.8 Macro.
