Help required to identify lens (or photographer) image quality problem

[Stephen Edgar]

Hi Folks,
I would appreciate some help in identifying an 'image quality' problem that seems to have appeared recently using a Canon EF 28-135mm f/3.5-5.6 IS USM attached to a Canon 5D body
I have attached four files, two 'original' files and a cropped section from each original.
I am puzzled why the quality of the pictures is so (technically) poor, particularly around the edges (and particularly the left edge) of the images?
The centre of the images seems fine.
I would appreciate some expert insight in identifying if there is a lens issue or if perhaps the fault is simply poor technique from the photographer
I must stress I am not a 'pixel peeper', but I am curious to understand the problem
Thanks in anticipation for any feedback,
Regards
Stephen Edgar

[Jeremy Roussak]

Aperture?

Jeremy

[Bryan Conner]

My guess is that the lens is decentered.  If you would like to investigate this possibility yourself, instructions (and advice) to do so can be found at http://www.photozone.de/lens-test-faq  . 

[David Sutton]

I took the liberty of looking at the metadata and your aperture and shutter speed look fine. So it comes back to a lens issue. I hear this lens is quite good on a crop sensor but some have shown problems on a full frame at the edges. It may be worth having it checked. They can also fail "out of the blue", as I've found to my cost.

[Stephen Edgar]

Thanks everyone for the input. It was appreciated!

Stephen