24-70 not sharp at 24

I bought a 24-70 a few years ago and shame on me for not testing it more thoroughly while it was still new. I typically shoot in the 40-70 range and this copy has been excellent; good sharpness, contrast, saturation, etc. I have a 16-35 so don't shoot at 24mm often, but did at a recent wedding and was disappointed with the sharpness. I thought it was me, but it is now clear to me that it's the lens. It has been calibrated to each of my bodies at 50mm.

Digging further, I come to find that this is not uncommon with this lens. Reading reviews, I see users talking about how they sent it out to Canon, but they typically do not say whether the repair was successful or not. And in my case owing to the time that has past, I'll need to pay for the service and I'm curious as to the cost.

Has anyone had experience with this and, short of selling the lens, how did you address the issue?

Thanks.

I remember someone else having this issue, and after sending it in a couple of times he escalated the issue because it was not getting fixed. He eventually received a new lens from Canon and the new one was tack sharp at all FLs.

I'm grateful my copy is extremely sharp throughout.

Most POTN users (including myself) require photo samples set at 24mm to validate sharpness.

As for my experience with this lens, here's how the story goes...

A few month ago, i bought a used 24-70mm (Date Code: US) from a local CL poster. During testing of the lens, it was heavily back focusing at 24mm, and required +20 MA. Even at +20 MA, the lens still experienced considerable front focus. I asked him if he has ever sent the lens to Canon and to my surprise, he provided a canon service receipt that states that the lens is fine. Even though there was a canon service receipt, the lens behavior was still questionable. I knocked the price from $1200 to $950 and went home with the lens.

After more testing, I was 100% sure that this lens was a horrid copy. I returned the lens to the original owner and my money was refunded. He then sent the lens back to Canon and Canon sent it back untouched. After receiving the lens, we met again, and this time, I decided to take full responsibility in repairing the lens. I knocked the price further to $850 and immediately sent the lens off to Canon. I sent them an extensive letter that accompanied the lens. Here's a couple of exerts from that same letter.

...When I tested the lens with each of my bodies, I found that the lens focuses just fine (but it is still not that sharp) at 70mm at f2.8. Unfortunately; I am experiencing a HEAVY front focus issue when using this lens at 24mm at f2.8. I have had 2 other copies (both of which I regret selling) of the Canon EF 24-70mm f2.8 L USM Zoom Lens and have never had this issue. I also have other lenses such as the Canon EF 35mm f1.4 L USM, Canon EF 85mm f1.2 L II USM, and the Canon EF 135mm f2.0L USM, and have never experienced an autofocusing problem when used on my camera bodies. This would then lead me to think that it is really an issue with the lens...

...I have emailed customer_support@canad​a.canon.com two sample photos of this lens when used with my 5D mark II. One photo was set at 70mm, and the other was set to 24mm. The photos easily show the focusing issue that I am facing...

...Please take the time to adjust the autofocus properly at varying focal lengths (24mm, 35mm, 50mm, and 70mm). I do not care how long it takes; I want this issue resolved. Please do not hesitate to give me a call at (XXX) XXX-XXXX regarding the autofocus issue with the lens. I can probably describe it better over the phone...

...This lens has already been sent in twice by the original owner. Every time that it has been returned, the issues still persists. The last time that it was sent in was on June 03, 2010. I really hope that this will be the last time that it will be serviced...

When canon received my lens, they emailed me an apology letter and said that they will fix this issue at no charge. After 4-6 weeks (I didn't care how long it took), I received the lens in the best condition possible. The lens was TACK sharp and required no MA form my camera bodies. According to the canon receipt, they replaced the PCB board and the focus assembly within the lens. The lens was also cleaned and calibrated.

Though it is my least used lens, I am definitely happy with Canon's service.

Thanks guys. I have a string of weddings coming up in October but then it's quiet for a few months. Although this lens is nearly two years old now (once again, shame on me for not testing), I'd like to think Canon will step up and still do the right thing. For those next weddings, I'll either need to swap to the 16-35 or do the focus-first-at-70mm-then-zoom-to-24 trick.

Mine is sharp at all FL's...I use a 7D but I don't think it makes any differences.

Good/bad copies I guess.

I had the same problem with mine when I got it new. It was sharp at 70mm and soft at 24mm. I tried adjusting it on my 50d, but when I got it sharp at 24mm, it was soft at 70mm. I sent it back to Canon with a very detailed explanation of the problem. A couple weeks later, I got my lens back. They said they replaced the collar, adjusted the alignment of the elements, and calibrated the lens. Now it is tack sharp throughout all focal lengths. I couldn't be happier with my lens and Canon's service. This was all done under warranty, so I don't know how much it cost.