18-135 Sharpness

[GenXRailmedia]

I've had this lens for a while, using it on my 650D, and I'm not loving the results.  I'm finding that at almost all focal lengths, the edges are just plain blurry.  I know that few lenses are sharp throughout the frame, and I don't expect "L" quality but what I'm seeing doesn't seem acceptable.  Yes, you get what you pay for, but even my old 28-80 USM was better than this 18-135.  (Sometimes I wish I would have kept the 28, even though it's not really wide enough for a crop body.)

The middle of the frame consistently is very sharp. I've tried stopping down (usually to no more than f/8) turning the IS on and off, and try my best to avoid camera shake. My shutter speed rarely is less than my focal length. Could my AF method have something to do with it? I usually stay on AI Servo.  I've tried manual focusing, but with the Rebel's smaller viewfinder, it's not easy and all of the times that I've tried MF, the results have been equally disappointing.  

Could I have just gotten a bad copy?  Decentering issue?  Or is something that I'm doing wrong?

[GenXRailmedia]

(I'm a little late getting back to this, it's been a busy week.)

Wadizzle, it was a bit remiss of me to not mention it in the original post, but this is the 18-135 IS STM lens.  

To answer your question about how close I was:  I went back and looked at it again.  100 feet may have been too much -- I was still a good 50 feet away.  

As far as as the focusing:  I looked at the raw file on DPP; the nose of the engine is sharp, the center AF point was just to the left of the nose of the engine.  

[GenXRailmedia]

Tim, there's a lot of good information in your reply.  I think I need to do a little more homework as far as DOF is concerned, but I'm starting to realize that I don't know as much as I thought I did.  

Once again, thank you and everyone for your replies and information.  

[Waddizzle · Accepted solution]

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@GenXRailmedia wrote:

Tim, there's a lot of good information in your reply.  I think I need to do a little more homework as far as DOF is concerned, but I'm starting to realize that I don't know as much as I thought I did.  

Once again, thank you and everyone for your replies and information.  

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Take a look at the link in my first reply.  It is a link to a DOF chart.