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See all the dark yellow speckles in the shadows?  Those are artifacts and noise.

No...the yellows should blend smoothly in with each other.  There also should not be any discolored spots or speckles in the green areas.  I used Neat Image on the after photo, but you can also use the blur tool for more precise smoothing.



Sorry this image is so small, but the file was too large to attach.

I see the spots Karimala pointed out, aren't they noise, though?  Sometimes lightening shadows I get that. 

Can compression cause this type of artifacting too?  I ask that because of the rejection reason Cricket got. 

Regards,
Adelaide

Two images for ya..

They are 100% crops out of a stock image of an old Medium Format camera..

The smooth looking one is a high quality jpeg.  The other is at setting 10 out of 100 in the gimp, which I suppose is the same as 1 or 2 in PS..

The smooth area of gradation around the inside of the lens no longer has smooth gradations, a the edges of the lines are effected by 'jaggies'  The smooth gray metal edge has developed a whole lot of 'noise'..

Does that help?

I was thinking the reviewer got it wrong when they called it "compression" artifacts, because it's clearly noise and regular artifacting that's the problem.  Noise reduction software can't fix true jpeg compression problems.

Download and view the attached images at 10x the size.

No tricks here, I created a white jpeg 150x150 pixels, with some gray diagonal lines on it..

1.jpg is at 100% (no compression).  2.jpg is at 50% setting in gimp (6 in PS?).

You'll see when you zoom in the 2.jpg shows 'noise' as well as the detail 'clumps' on the lines.

This is because jpeg replaces like chunks of data with either a solid lump of one color, or a gradient between two colors that were similar.  For the 'blocks' of white in between the gray bits it replaces them gradients approximating the gray to white transition present near the gray line.

That's a very rough idea of what it does of course, as the jpeg algorithm is actually very complex, but you can see the result...  Have a play in PS with your own images..

Anyway, for stock the same old rules apply...  Shoot in best quality jpeg or raw at a low ISO (Let's not start the raw vs jpeg thing all over!) save partially edited files as uncompressed files (native ps or GIMP format, tif if you really must), and save the jpeg at 100% or 12 () in PS.