OK, I have been sent two cropped images by Tim. Here is my response to him:
I must admit I am puzzled. Yes I can clearly see the artifacts.
I've tried the same process as you and I have to admit I see similar (though not so noticable) artifacts in a picture of mine.
(1) they are not obviousy present in my original jpeg, only those saved by photoshop - even a crop represents a resave of the original image (yes?) so the only true original is the image out of the camera without a crop
(2) they are not noticable if the image is reduced to smaller sizes (i.e. 1024 pixels)
(3) they are not in any way noticable in a photo quality print from the full size original image at about 12x8 inches
I've not bothered to send my own findings (unless you really want to see them). However, I use broadly similar settings to you - 100 ISO, usually aperture priority F8.0, large fine, etc. on the Canon 100-400IS or 28-135.
The only conclusion I can draw at this moment is that the artifacts are put there by photoshop - I cannot see them in my image in the original jpegs captured by the camera without any processing/cropping/saving etc. I can only assume these artifacts are caused by the process of saving a jpeg as a second generation - as I say, even a crop followed by a save is a second generation, and therefore subject to some degredation. Maybe raw will work better?
You should ask Colin Work - ckw - he's an expert in such matters. Perhaps forward my comments, and maybe let me see what he has to say too.
Sorry I cannot be of more help,
Colin, I hope you don't mind me recommending your opinion!
There are old pilots and there are bold pilots, but there are no old bold pilots