This presents itself as a lighter area appearing to be overlaid on the image. 1. It's only with one lens, Nikon 18-70. I have four other lenses and it does not occur with any of these. (18-55, 55-200, 100mm macro, 70-300) 2. The area is always approximately the same size and in the same position, though it does seem to become more defined as the lens stops down. 3. It occurs with both my Nikon D70 and D200 bodies. 4. This is the second lens of this model in which this occurs. I ditched the first because of this problem! 5. It appears on all images, though it is more or less noticeable depending on the underlying image colour. Most obvious on a dark background. 6. It appears on both RAW and JPG files. Attached is just one image that shows the problem. The artefact is in the middle vertically towards the right edge of the image. Any suggestions gratefully received.
As you don't mention anything about cleaning? Did you check for dust for instance? I have had a similar problem where 'something' was attached to the inside of the glass which led to artifacts. After a service this has been resolved.