I recently started having focus issues with my most-frequently used lens, the Canon EF 24-70mm f/2.8L. It would often rear-focus a lot, despite using only the centre focus point and single-shot focus mode. This rear-focusing behaviour didn't occur all the time, and it seemed to occur more frequently at wider focal lengths.

There was also some in/out movement in the lens barrel that I previously hadn't noticed, and I suspected that this movement was related to the focus issues.

Canon EF 24-70mm f/2.8L lens

Here's an example photo, showing the bad rear focussing behaviour. The photo was taken using the centre focus point only, and single-shot focus, focusing on the face of my oldest son.

photo straight out of the camera

While the rear-focusing isn't particularly obvious in the scaled-down photo above, the 100% crop below shows it much more clearly.

100% crop showing rear-focusing

The 100% crop shows that the lens has badly rear-focused, with the kids' faces out-of-focus, and the rigging and deckhouse in the background is in focus.



I remembered reading some interesting focus test results on the Canon 24-70mm lens some time ago, and found the lensrentals.com blog post (scroll down to the section titled "The 24-70 mystery").
In that post, lensrentals.com explain how they have noticed that the focus accuracy of the 24-70mm lens degrades over time, due to plastic collars in the barrel tracking mechanism wearing and falling apart.

The symptoms matched what I was seeing, and worn / broken / missing plastic collars also explained the movement I was experiencing in the barrel of my lens.



My 24-70mm lens was purchased new, about 3.5 years ago, so it was no longer covered by Canon's warranty. I dropped my lens off at my local Canon retailer, and it was sent to Canon for investigation.

It was about 4 weeks before I got the lens back - and as expected, the plastic collars were the issue. At AUD$485, it wasn't cheap, but because I use the 24-70mm lens so much, I figured it was worthwhile repairing (unlike my earlier experiences with the cheaper 17-85mm lens, which I repaired myself).

These repairs have also eliminated the movement in the lens barrel.

Testing with my newly repaired 24-70mm lens confirms that the focus is now operating correctly under a variety of conditions. Who knows - maybe the focus performance will be better than when new (I've previously written that I wasn't overly happy the performance of my 24-70mm lens).

Hopefully Canon have modified the collars and internal construction to address this issue in the recently-released and updated 24-70mm f/2.8L II lens.