Please help

I just purchased a used Tamron 24-70 vc lens. when i received the lens. I did what all of us do an begin testing sharpness and focus speed/accuracy.

Here is my problem:
When using the outer AF points at max aperture, the lens has severe back focus. However, when i switch to center point the lens is tack sharp. I have a series of shots that i took with upper right af point-center af point- upper left af point- back to center af point with the center point being tack sharp and the outer points being extremely soft. i am using a 5diii. Could someone please tell me what this defect is called? I am trying to get my $ back on this purchase.

Also, does anyone know if the zoom lock switch is only supposed to work at 24mm.

Those corner shots could be non-sharp for a variety of reasons.

Were you focusing with live view when you took the corner shots?

Don't blame the lens for something that might be caused by your camera.

no i was using the viewfinder

Then it's highly likely that the 5D3 let you down.

Most likely the defect is decentering and has nothing to do with the 5DIII. Normally, if it back focuses in some outer point and it is tack sharp in the center, it will probably front-focus in the opposite outer point. If you do these tests in Live View, on a tripod, it will be fairly easy to document decentering.

If you can shoot a brick wall with a tripod and center point, making sure that you can capture some grass in front of the wall, you will see the focal plane tilting from one side to the other.

My first EF 24-70 MkII acted worse than this and returned it within 14 days. The second copy is perfect.

From my so far non scientific testing (handheld), it seems to back focus on all outer points. The points surrounding the center are sharpish. I mounted it to my 6d and the lens seems fine from my again, non scientific testing.

If this is body related, why do my other lenses work great? Can this be corrected?

Yes, you can micro-adjust the AF on your 5D3.

amish wrote in post #17443830
From my so far non scientific testing (handheld), it seems to back focus on all outer points. The points surrounding the center are sharpish. I mounted it to my 6d and the lens seems fine from my again, non scientific testing.

If this is body related, why do my other lenses work great? Can this be corrected?

Are you sure it's back focus, and not just soft at the outer points? I have a hard time figuring out what kind of lens problem would give the symptoms you are describing, other than just being soft in the corners.

The lens is sharp in the corners on the 6d. If I micro adjust the AF points on the 5diii because the center would then be out.

What you're describing sounds like field curvature (which would cause the center and edge of an image to focus at slightly different positions) - a known issue with the Tamron and the Canon mk I 24-70 lenses.

This was my original thought as well GregDunn. I thought i just had an extreme case of it. However, i cannot explain why it works great on my 6d both at the edges, and center af points. Ugh... so frustrating. I feel that i could go out and confidently use this on my 6d but not on the 5diii. I am wondering if i should send my 5diii in for inspection?

Amish, I went through several copies of 24-70 vc, before I got one which funcitons perfectly on my 5d3. by perfectly I mean, well centered, no VC issues, and accurate and consistent focus from all focus points. I had a well centered copy which had the exact problem yours does. Mine however focused accurately will the the vetical center points (up and down), but not if I moved laterally (to the sides).
I sent it in to Tamron, they sent it back, maybe slightly improved focusing, but much worse optically. Went back in a second time, without much improvement. went back in again, and I spoke to a rep which was very helpful and in constant communication with me. they tested the lens extensively, tried to fix it on my camera, could not, and then tested 3 different brand new stock lenses, before they found the one I have now.

The rep told me the AF of 5d3 is complex and problematic, whatever that means. I dont have an issue with my canon lenses.
So this one is great. Ironically, in the process, I was so frustrated that I bought the canon 24-70 2.8L II lens. I am having a hard time parting with tamron, b/c It took 8 lenses before I got this, b/c of VC, b/c its optically about equal to my canon. The canon is sharper at 70 2.8, if you look at 100% crops. Canon does better in the corners on the wide end, and tamron actually slightly better on the long end. The canon however, has blazing fast AF and also seems to do a bit better in low light with AF.

Hope this helps. So, you can return it and get the canon, send it in and roll the dice, but you have to be ready to spend a lot of time with repeat testing to make sure you have a good one, or just use it on your 6d.

thanks for the comments Kevindar. That's just the thing as well. I didn't seem to have any issues with my canon 24-70 f2.8 L before i decided i wanted to try the tamron vc. I figured it was going to bite my ass and here it is. I did not buy new. I purchased used telling myself that was a bad idea. I won the argument with myself knowing that i had purchased several other used lenses that were just fine. I still cannot think why this lens works great on my 6D. ... or does it... maybe i haven't done enough testing. The person i acquired the lens from stated that it worked perfect on their 5diii.

One more question for you Kevindar (and for anyone that has this lens). When performing lens corrections, does it show up as a Sigma lens or a Tamron? mine shows up as a Sigma.

if i can figure out how to post images on here again.. I'll add some or maybe link to my dropbox so you guys can check them out for yourself.

lightroom defaults to sigma. I dont think thats a reporting issue, its a lightroom issue, if thats what you are referring to.
As for the used lens, you may have bought from a pefectly honest seller. I went through my first 5 copies, looked for a properly focusing and well centered one, and tested them a lot, and was happy with its sharpness. then I shot an even with it, and there were a lot of OOF shots, b/c I was hooting in portrait orientation, and the non center focus point, which led to off center focus testing. thats when I realized the problem. As it stood of course, the lens was not usable, b/c I mostly used to for people photography, and needed off center focus accuracy.

Good luck whatever you do. I would offer you my lens, but you already have one to deal with, and I cn not in all earnest gaurantee it funcitons perfectly on your camera, just that it does on mine.

Yeah thanks Kevindar. I'm shocked that I havent heard more about this form other users. I have a friend here at work that also has a 5diii. I'll see if he'll let me mount the lens to his body and see if there is a difference in af performance away from the center of the frame.