T5i and Sigma 17-70 focus problems

[DavidSE8]

I purchased a T5i last December as an upgrade from my Rebel XT.  Comparing the two cameras with the same lens the T5i has a definite focus problem compared with the Rebel.  I have returned the camera to Canon for repair, only to be told that there was nothing wrong with the T5i, that it must be the lens.  If that is so, why is the focus so much better on the Rebel XT?   I have been told there might be a chip in the lens which is not somehow compatible with the T5i body, but I have never heard of such an issue.  Could I perhaps have some obscure setting on the T5i body set incorrectly?  Any help would be greatly appreciated.

[RobertTheFat]

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@DavidSE8 wrote:

I purchased a T5i last December as an upgrade from my Rebel XT.  Comparing the two cameras with the same lens the T5i has a definite focus problem compared with the Rebel.  I have returned the camera to Canon for repair, only to be told that there was nothing wrong with the T5i, that it must be the lens.  If that is so, why is the focus so much better on the Rebel XT?   I have been told there might be a chip in the lens which is not somehow compatible with the T5i body, but I have never heard of such an issue.  Could I perhaps have some obscure setting on the T5i body set incorrectly?  Any help would be greatly appreciated.

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What exactly is the "definite focus problem"? If the lens consistently front-focuses or back-focuses (Google if you're not sure what that means), then the lens and camera body are relatively out of adjustment. Some Canon cameras have an autofocus microadjustment feature that can often correct such a situation. Alas, the T5i, like all Rebels, is not one of them. So the adjustment, if it can be done at all, would presumably have to be done by a professional repair facility.

[jrhoffman75]

There are chips in the lenses - its what controls the autofocus and aperture movement. Whether or not there is a compatibility issue with T5i you would need to contact Sigma.

Do you have or can you borrow a second lens? If so, how does it focus with the T5i?

[TCampbell]

Assuming your focus tests were valid (I'm pretty fussy about what constitutes a valid focus test because SO many things can throw off focus that have nothing to do with the camera or the lens) then you may have stumbled onto an incompatability issue.

This is one of the possible issues when using 3rd party lenses.  

Canon doesn't publish specs on how the bodies communicate with the lenses.  The lens makers reverse-engineer that.  They test their new lenses for compatibility only with camera bodies that are currently on the market.   

Unfortunately that means that if you buy a 3rd party lens for your then-current camera... and years later you buy a new camera, your old lens will never have been tested for that new camera (which didn't even exist when you bought the lens) and you can find compatibility issues such as the focus issue you are currently having.

Canon makes sure that all camera bodies are compatible with their entire range of Canon lenses -- but that's because they're Canon and they know every lens they've ever made and they know the detailed specs on how the body and lens have to communicate.  

You should contact Sigma regarding the issue.  It's possible that you may be able to send the lens to them and they can update the firmware for compatibility with your new camera.

[ebiggs1]

David,

There has been three versions of this Sigma lens.  You need "C". The latest version.  The Sigma 17-70mm f2.8-4 is never going to make outstanding shots at the wide end.  It does get much better as you zoom to the longer focal lengths.  Actually somewhat better than the Rebel kit lens of this type.

Sigma can update your lens if it is not the latest "chip".  I don't know what that costs, you may want to check it first.

The Canon EF-S 17-55mm f/2.8 IS USM Lens is a much better lens and only a little more expensive.  Plus it has the distinct advantage of being a constant aperture and is made by Canon especially for Rebel cameras.