Problem with the middle auto focus on canon 6D

[andregrafie]

Hello guys,

I discovered a problem with the middle auto focus point on my canon 6D. First of all I thought my Sigma 50mm 1.4 ART Lens is responsible for it, but I tested it with another lens and with the canon rebel t3i.

The result:
When I use my canon 6D and my Sigma 50A or the canon 50mm 1.8, the focus is inconsistent, it jumps! some shots it is more in the back, on some shots it is more in the front.
But the problem only appears when I use the middle point of it, the cross sensor (hope that is the right word in English). When I use some of the others, everything is fine!

When I use my canon rebel t3i, I don't have this problem, everything works fine here too.

I own the Canon 6D for not even 2 months, maybe the problem appeared from the very beginning, I am not sure, I haven't tested it before.

What could be responsible for this problem?

sorry for some English mistakes, it isn't my native language

Thanks,
André

 

[ronlane]

The 6D should have controls in the menu section for you to micro adjust for lenses that are doing this. Check your manual and search youtube for how to adjust it.

 

[andregrafie]

as far as I see/know is, that this micro adjustment only works with constant back or front focus, not with inconsitent AF. I mean, let's say I adjust the lense +5 and then sometimes I have back sometimes a front focus, because it jumps, where it wants to.

 

[ronlane]

Oh okay. I personally would get it back to the store or to canon, since it is only 2 months old. It should be under warranty, so that is a good thing, other than having to be without it while it is repaired.

 

[andregrafie]

yes I also considered that, but I want to be sure, that his problem, is actually a problem, not something, that is normal...

here 2 photos, that shows, how inconsistent the middle AF works:

You can see it pretty cleary on the left and the right battery, I used a tripod, AV-Mode, AWB, ISO 100, f/1.4

and again: I don't have this problem with the other AF-points, or with any points of my rebel t3i

 

[ronlane]

So at f/1.4, you focused both of these images on the middle battery, correct? I don't think that I can say why it is switching the sharpness from in front to in back of the middle one but here is something else for you to consider.

At f/1.4 the depth of field is really small so the other two batteries should be really soft.

The difference in the depth of field for a crop sensor versus a full frame is going to be a LOT different, so saying that the issue doesn't show with the T3i, isn't a fair comparison.

I still personally think that this is an issue with the lens and camera not being in sync. But I am sure that there are others here that could weigh in on this, that have more experience than I do.

 

[andregrafie]

yeah sure, I considered the full frame and crop factor, when I for example did the test with the t3i, I arranged the distance.

But again: I don't have the problem with the other focus points just with the middle one AND I have the problem with the Canon 50mm 1.8 as well. So it definitely has something to do, with the canon 6d middle point, the middle point of the t3i is fine.

 

[TCampbell]

Inconsistent focus usually indicates a problem with the lens... not the camera (and there are some technical reasons why this should always be true.)

You do need to take particular care when performing a focus test to make sure you're not being misled by your results.

You will want a focus target with a scale that will help you identify if the lens is front-focusing vs. back-focusing (vs. nailing the focus correctly.)

I use one of these:

You can see that the target has a nice easy high-contrast image to help the camera lock focus (so no excuses for why it wouldn't be able to focus accurately) and then on the right what might be a bit more difficult to see is that there's a scale which is actually on a 45º angle leaning away from the camera... but the "0" point on that scale is exactly even with the flat focusing target.

This way, if the camera misses focus, we can tell if it front focused or back-focused and we can even tell how much (those are centimeters marked on the scale.)

Also notice this target is stationary (on a tripod) and what you can't see is that the camera is ALSO on a tripod. This way there's no possibility that I locked focus while hand-holding the camera... then then my body slightly leaned forward or backward by so much as a centimeter.)

When I perform a focus test, I do the following:

1) This first step is perhaps a bit unconventional in that I've never read an article telling me to do this, but I have found that it works well...

I LOOK a the manual focus ring on the lens and I find the focused distance which is at the mid-point of the focus ring. For example, I'm staring at the 50mm lens on my old 35mm film camera (because my DSLR is not handy right now but the 35mm SLR is) and ... even though the focus ring shows distances from roughly .6 meters (2 feet) to infinity, when the focus ring is turned halfway between minimum and maximum focus distances... the value indicated on the focus ring is actually about 1.2 meters (4 feet). So I set my focus target about 1.2 meters (4 feet) away from the camera.

2) Set the camera to aperture priority mode and then set the aperture to the minimum possible f-stop (largest possible aperture size) to minimize the depth of field and make it easier to see if you are achieving proper focus. You can use Manual mode ... but select the lowest f-stop and then set your shutter speed accordingly. ALSO... Make sure you are in "One Shot" focus mode (do NOT use AI Servo mode or your test isn't valid.)

3) Manually focus the lens to the MINIMUM focus distance (deliberately rotate the focus ring so that the camera is not focused on my target).

4) Allow the camera to focus on the target and take the shot.

5) Repeat steps 3 & 4 AT LEAST a half-dozen times. (and NOTE which images were taken with lens starting at MINIMUM focus distance.)

6) Manually focus the lens to the MAXIMUM focus distance.

7) Allow the camera to focus on the target and take the shot.

8) Repeat steps 6 & 7 AT LEAST a half-dozen times. (and again... NOTE which images were taken with the lens starting at MAXIMUM focus distance.)

Now transfer the images to your camera and inspect them for focus accuracy.

The reason for running focus in to the minimum for half the shots and then out to maximum for the other half of the shots is to determine if you've got gear backslash (slop) in the mechanical focus system within the lens. This would result in the focusing elements not stopping in the correct position when the camera focuses.

If the lens doesn't have any mechanical slop, and the camera is missing focus, then the camera should show a fairly consistent pattern... perhaps it always focuses just a few millimeters too close for example.

Since the camera isn't moving and the target isn't moving, most all of your shots that started with the lens at "MINIMUM" focus distance should land in a consistent place (you may have the occasional outlier... but if it's just an isolated outlier then you can ignore it.) Also, most all the shots that started with the lens at MAXIMUM focus distance should also have the focus land in a consistent spot and if there's no gear backlash problem in the lens, that SHOULD be the same location as the shots that were taken starting at MINIMUM focus. If you get different results for the lens based on whether it had to run focus inward vs when it had to run focus outward then YOU DO have a gear backlash problem with the lens (it's the lens... not the camera).

Since your focus target is consistent, the distances are also consistent, the camera is NOT hand-held, etc. then your results should be consistent. There are no moving parts to the focus system within the camera (that's 100% electronics -- that will be consistent.) The only moving parts in the focus system are within the lens. This is why random focus error is almost always an indication of a problem with the lens and if your results differ it makes me worry that perhaps you are not carefully controlling your test conditions -- you may be introducing other error that isn't really the camera's fault. You want to make you isolate factors that could introduce error.