L lens woes...(Back Focusing/Need Opinions)

[Austin Greene]

EDIT: I should have included that my body, the 550D, does not have an AF micro-adjust feature. If it did, I wouldn't be making this post.

Well, I have to say that I was incredibly excited over the weekend to have acquired my first L lens, a beautiful 70-200 f/4 IS. Bought used over CraigsList, I did a thorough investigation of the lens, running through an entire checklist including everything from checking for dust in the inner elements, to inspecting if the mounting plate screws were worn or stripped. I also attempted to check for back-focusing, but apparently didn't do a good enough job. Damn if I didn't take 30 minutes giving the thing a once over, and taking shots with it (albeit in low light, it was early evening) before deciding to buy it.

Fast forward to today, and as I'm shooting my usual wildlife subjects, I begin to notice something. The photos aren't sharp, at least not where they should be. Everything is back-focused by roughly a centimeter. I have a hard time understanding it, the lens had only been used for about 100 photos, and had never been dropped. There wasn't a single nick on it. I drive home, and run some tests, still appears to be back-focusing. Lastly, I ask a friend to stand in front of a flash for me, so I could test the lens in a real-life situation. Focusing on her brow, centered between here eye-brows I take the shot. Sure enough, while her brow is just barely coming into focus, her eyes and the skin around them (in the sockets) are acceptably sharp. Back-focusing again.

I sit on a lens which is functionally useless for all its beauty and hype. That pretty USM AF motor doesn't do me any favors if it consistently focuses 8-10mm behind my intended point. The lens is three years old, and well out of warranty.

I have two options:
1) We have a local place, which has been recommended to me by a local shop as a possible repair site as they have a reputation for re-calibrating lenses. I will be calling them tomorrow for a quote.
2) I called Canon, and sending it to them is an option. That said, they would be a far slower than using the sacramento shop, and I have serious questions about the whole "send it to us, and then we'll tell you how much it will cost" process. I simply can't afford another couple hundred dollars.

So here I am, totally confused on what to do. Have any of you had any experiences with Canon's services? Are they decent, did they actually fix the issue? Is the 1 week turnaround time they told me a reliable estimate? How much did it cost for you?

Also, has anyone in the Sacramento valley had an experience with the mentioned shop? One of my concerns is how they go about calibrating lenses (they used to be a licensed Nikon service center awhile back), and if it could mess with the weather sealing on the lens. If it won't, do I have anything to lose by going through them, assuming they could get it done right?

Thanks for all the help, I'm pretty well depressed at the moment.


Here is an image for your verification. As mentioned, it was focused dead between her eyebrows, and area which is clearly unsharp, while an area slightly farther back (skin by her eyes) is acceptably sharp.

Link for full image

 

[subscuck]

Just some observations: You're shooting wide open in this pic, not the way to test sharpness. Are you using center AF point only? If not, how do you know your point of focus is right between the eyes? I would set up on a tripod and shoot a ruler or newspaper page, select center point only. And stop down a bit, at least a stop or two, to evaluate sharpness. I wouldn't look into having it serviced til you're 100% certain you have an issue.

 

[pgriz]

The AF backfocusing is not a lens thing (usually), it's more related to the location of the AF sensor and its optical distance from the lens. If the sensor is exactly at the same plane as the sensor, you should have no issues. But if they are not, then you get front/back focusing. I had that issue, sent my photos to Canon, they agreed with the diagnosis of the problem and had me send the camera in for adjustment. It took about two weeks, and the AF focus was much better (although still not perfect). My camera (because it's a camera thing, not a lens thing) was still under warranty and I didn't have to pay. Don't know what it would be in your situation.

Subscuck, I need to disagree with you. If you are trying to diagnose backfocusing issues, then shooting wide open is the best way as the DOF is the thinnest. However, for regular picture taking, I agree with the advice, as it will generally place the lens close to its "sweet spot", and the deeper DOF will cover "most" focusing errors.

 

[gsgary]

I have never had a problem with any L lenses i have bought most of the time it is user error, where you using a tripod ? if no how do you know you didnt move

 

[bratkinson]

I just purchased and used FoCal software from ReiKan to perform micro focus adjustment of my lenses. I did 4 of the 5 Ls I own and the 85mm f1.8. My MFAs varied from +2 to +7 on my 2+ month old 5Dm3. When I had the 60D, I thought they were a tad off, but still produced WOW results. I haven't had the chance to 'test drive' the new settings. Hopefully this weekend.

As Subscuck noted, shooting wide open won't produce the sharpest images from your lenses. Also, your DOF gets incredibly thin wide open, close to the subject, as your picture appears to be. Shoot a stop smaller and your nose/eyes/bridge of nose DOF issues will be gone.

For what it's worth, I did a full 'sharpness test' on my 135 f2L using FoCal and, as expected, there was a noticable loss of sharpness at f2.0 But at f2.2, it was noticably sharper, almost to its' most sharp at f4.5 as I recall (I should have saved the results!). The 135 now has an MFA of +5 on my camera.

Bottom line, each camera focuses differently with the same physical lens. Even different copies of the same camera model with the same lens may be different. You may ultimately find that having a non-MFA-capable body adjusted for that single lens may throw off any other lenses you (may) used with that body, so beware of the pitfalls.

 

[Austin Greene]

Just some observations: You're shooting wide open in this pic, not the way to test sharpness. Are you using center AF point only? If not, how do you know your point of focus is right between the eyes? I would set up on a tripod and shoot a ruler or newspaper page, select center point only. And stop down a bit, at least a stop or two, to evaluate sharpness. I wouldn't look into having it serviced til you're 100% certain you have an issue.

As was mentioned by pgriz, I was shooting wide open for a reason, that being to get the thinnest DOF possible. The issue is not diagnosing if the lens was back-focusing, that has been confirmed by local shops and numerous tests, it was taking a photo which could communicate it to TPF. This was the last of three seperate tests, all ran off of a tripod. The first was the classic ruler at a 45 degree angle, the second was a test using a printed chart, and this third test was in a more realistic situation to see if it would actually create an issue. As pictured, it would.

The center AF point was used. I should have rlaborated more in the initial post, maybe I expected more confidence from folks in my abilty to determine the issue. Stopping down or using multiple points would hardly address the issue.

I have never had a problem with any L lenses i have bought most of the time it is user error, where you using a tripod ? if no how do you know you didnt move

I was using a tripod, and was meticulus in my testing. The issue was confirmed to be with the lens after testing every one of my lenses and finding no issues across multiple styles of tests. Even though the original owner swears otherwise, i suspect it was dropped or shaken up somewhere along its three year history of backpacking. Also, the owner used it primarily for landscapes (chalk it up to personal preference), so I suspect focusing issues would have been less apparent.

Regardless, the lens has been sent to Canon. At the worst, if somehow it is in fact fine, I figure it couldnt hurt for it to get a good cleaning/internal once over since I plan to use it well into the next decade. They will be calibrating it to my specific body.

 

[Austin Greene]

The AF backfocusing is not a lens thing (usually), it's more related to the location of the AF sensor and its optical distance from the lens. If the sensor is exactly at the same plane as the sensor, you should have no issues. But if they are not, then you get front/back focusing. I had that issue, sent my photos to Canon, they agreed with the diagnosis of the problem and had me send the camera in for adjustment. It took about two weeks, and the AF focus was much better (although still not perfect). My camera (because it's a camera thing, not a lens thing) was still under warranty and I didn't have to pay. Don't know what it would be in your situation.

Subscuck, I need to disagree with you. If you are trying to diagnose backfocusing issues, then shooting wide open is the best way as the DOF is the thinnest. However, for regular picture taking, I agree with the advice, as it will generally place the lens close to its "sweet spot", and the deeper DOF will cover "most" focusing errors.

This is very interesting to me. Each shop and Canon support expert I have spoken to has consistently stated that focusing issues are typically more likely to be the product of lens issues, rather than body issues. Not to say that they can't occur involving the body, but the general impression I was given is that lenses are largely more susceptible to getting out of alignment. It will be interesting to see if the issue is fixed upon the lens' return, or if it will require work on the body. That said, i have never had any issues with any of my other lenses, and I am extremely sensitive to focal issues.

 

[pgriz]

We gotta be careful here. If we are talking purely focusing issues, then that's lens. If we're talking AF, then usually it's body, as I understand it. I will not claim to be an expert on camera mechanics, but what I've read on the subject refers to the calibration of the AF sensors so that they are at the same distance from the lens as the sensor. I have not disassembled my camera to verify this. However this implies to me that the actual physical sensor can be moved so that it can be at the right distance from the lens.