17-40L Back Focusing on 1D MK11?

[paulgunther]

Has anyone had the same experience as me:
I have a problem with back focus, but only when my 580ex is connected and the lens is set to17mm with multipoint AF selected.
When i choose center point the 1D MK11 focuses perfectly. With flash switched off, focus is perfect on center point and multipoint.
On 40mm there is no problems at all.
The body, lens and flash have been back to Canon Service for adjustment, but still the same problem, to make things worse, my other 1D MK11 set up, again with another 17-40L and 580ex has exactly the same symptoms.
The back focus is so bad that when you focus on a face or object from 1.5mtr the focus is correct at 2.5mtrs (but only on multipoint AF)

Regards
Paul G

[PacAce]

Do you have a sample picture that we can take a look at? I have a few ideas but I wanted to see an example of your back focus problem first and also of one that is OK, just for comparison purposes.

[mbze430]

That does sound strange. Because the 580EX AF assist beam will cover all the 45 points. I don't have a 1DMKII, but I do have a 1V, they are the same focusing system, with my 15mm, the farthest left and right point will pin point focus on my 1V + 580EX. Now you got me worry, because I am in the process of getting 1DMKII to replace my 20D.

[tim]

I thought multi-point AF was for people who used green box mode. If you want to focus on a single thing, use single point focus. Like Leo said, post a pic, preferably one that shots the AF points lit.

[mbze430]

On the 1-series there isn't a Green Box mode :). There is the P mode, Tv, Av, Bulb, and Manual. However the AF system can be set to full-auto or single cell mode (with variation). 45points, 11 points (2 mode) and 9 points. so in respect, in AF mode, it should cover all the points. You might want to put something in high contrast in the edge of the last focus point and see if it will pick it up.

[tim]

Doh! That makes sense. My point was though that perhaps the system has chosen the wrong AF point to fix on, and a narrow DOF isn't taking in the object that you want in focus.

[slin100]

It's rather hard to judge DOF on a 17-40. According to http://www.dofmaster.com, the DOF of the 17-40 is huge at 17mm at all but the closest of distances--several feet or more. The focus error would have to be huge. Make sure to rule out camera shake before blaming it on focus error.

[paulgunther]

Do you have a sample picture that we can take a look at? I have a few ideas but I wanted to see an example of your back focus problem first and also of one that is OK, just for comparison purposes.

I have enclosed 3 pictures, first "backfocus" is a crop of the one showing backfocus problem on the scale, second "correct" with center point, "point selection" it is full frame of the first one with chosen points selected - it is enclosed in the next post.
It may not seem bad on these pictures but on peoples faces it looks worse.

Regards
Paul G

[mbze430]

I don't see the image, I am going blind!

[paulgunther]

Do you have a sample picture that we can take a look at? I have a few ideas but I wanted to see an example of your back focus problem first and also of one that is OK, just for comparison purposes.

Ok, this is the 3rd picture "point selection"

Regards
Paul G

[PacAce]

The AF assist beam of the 550EX (and I'm sure the same must apply to the 580EX as well but you can check your flash manual to verify this) is only effective with lenses 28mm and longer and that may explain why you're only having problems when the lens is set to 17mm. And the reason why focus is work when only the center point is used is because the center AF point is much more sensitive than the other AF points on the 1DmkII.