Back focus on Canon 85 1.8?

[carlyfan]

Has any one else had issues with their 85 1.8 back focusing? I got the lens a few months ago, and as I've used it more I've noticed it seems to be back focusing. I don't know if it's getting worse or if I'm just noticing it more. It doesn't seem to be consistent though. Sometimes the focus is right on and sometimes it's way off. I use the center auto-focus point which I have heard is supposed to be the most accurate. Once in a while I also get pretty bad chromatic aberration. I can post examples if needed. I'm just wondering if that's that way the lens is, or if I should try to send it back and get a replacement. Any advice would be most appreciated. Thanks!

Lisa

[inkista]

Back focusing, no. It could be your lens and body are both out of AF calibration. If you have a 7D or 50D, you can adjust this yourself. But the dRebels and the 60D don't have this feature.

C/A (of the purple fringe variety) when shot wide open and on highlights, absolutely. It's typical of most fast lenses, although the 85/1.8 seems a little more susceptible than the average. Stop down and avoid blowing out highlights, and you can pretty much eliminate the fringing.

[carlyfan]

Thank you so much inkista. I have a rebel XT. So if they are out of calibration is there any way I can fix it? Is there any way to find out if it is my camera, lens, or both?

[inkista]

Only way is to send 'em to Canon service.

The thing is, it could also be user error, particularly if you're shooting wide open all the time, which means your accuracy has to pick up considerably to get a sharp shot where you're focusing. Unless you've tested on a tripod with a test target, eliminating camera shake, insufficient light, low-contrast target etc. etc. as possible causes, it could be technique.

If you are using the full AF grid, the AF system might be choosing the wrong spot to focus on.

If you are using a single AF point, and doing a half-press and recomposing, the recomposing might be creating the error, since the distance to the subject will change when you change the angle, and with an 85/1.8 wide open on a close subject, you might only have a DoF measurable in millimeters. Try selecting the point you need, without recomposing.

It might be camera shake. Are you shooting with shutter speeds at 1/100s or faster? Is your handholding technique good?

Have you tried using liveview and 10x magnification with manual focusing?

And have you tried stopping down?

Just thoughts.

[carlyfan]

It could be partly my fault. That's what I thought it was at first, but after using it more and more I don't think that's all it is. It was really acting up yesterday. Here are some examples. My sister was helping me set up a shot.
So I got one picture like this:

Then the next three looked like this:

They were both shot at 1.8 but I though I was far enough away that my DOF shouldn't have been too small. In both shots I was using the center AF point and focusing on her face.
This one was shot at f/4 focusing on the girl on the right. The focus appears to be on the parents.

I probably should have stopped down more, but even so, the girls should be in focus and the parents out of focus. In all three pictures I locked in the focus, recomposed, and took the picture.

My camera doesn't have liveveiw, but I have tried manual focusing at times when it's been acting up and it does seem to help.
Should I try to get a replacement lens? Could that help?

[inkista]

It might help to get another copy, if you're sure this one is bad. But my guess is it's user error. You're front focused on the first shot, and back focused on the second. That's not an AF calibration error--calibration error would be consistent on all your shots. Look at the grass. That tells you where you focused.

I'm thinking maybe there's some kind of mind/fingers disconnect on the focus/half-press thing. And you're shooting in lower light and wide open which won't help a lot.

You aim the center point at your target. You half press to achieve focus, the AF point lights up red. You then have to keep the shutter button half-pressed while you recompose. The AF point must stay lit the entire time. Then you reframe, and push the shutter button the rest of the way down.

If you release the shutter button at any point in the recomposing sequence, you'll lose your focus lock.

You may want to consider setting focus lock to be the default, instead, if you think you might be accidentally letting the button go. WIth the camera's custom functions, you should be able to do that. Custom Fn. 04, if you set it to 1 (AE lock/AF), I think (I'm not sure), will set the shutter half press to be your AE lock, and your * button on the back to be "start autofocus". That way, your default state is locked.

You aim at your subject, you press the * button until your subject's in focus, then lift your finger off the * button, and your AF is locked. Less fatigue in your forefinger than holding the half press.

[carlyfan]

So if it was a problem with the lens it would be consistent? There wouldn't be times when it worked just fine? Maybe it is just me then. I'll do some testing when I get a chance at different f-stops and distances. I'll also shoot some with my 50 1.8. If it is user error it would happen with the 50 as well, right?

One more thing, if the lens was back focusing would the manual focus be affected?

Thank you for your patience and your help! You're awesome!

[inkista]

Originally Posted by carlyfan

So if it was a problem with the lens it would be consistent? There wouldn't be times when it worked just fine? Maybe it is just me then. I'll do some testing when I get a chance at different f-stops and distances. I'll also shoot some with my 50 1.8. If it is user error it would happen with the 50 as well, right?

Actually, not necessarily. All lenses and camera bodies are made to a tolerance range (say ±3). [These are made-up numbers, btw:] So, say your body is -2, and the 85 is +3, both are within tolerance, but your total out of whackness is 5 and you have issues. But if your 50/1.8 is -1, then the combo's total out of whackness is only 1 and so it works fine together. Kind of like that. If you send the 85 and your XT in to be calibrated, they'll both be set to 0. It may mess up some of your other lenses, which is why some folks recommend sending everything in all at once. Or buying a camera with autofocus microadjust so you can adjust yourself.

Generally, I'd say if it is the 85, just send in the 85 and see if it works better when it comes back, and only worry about overall system calibration if that doesn't work.

One more thing, if the lens was back focusing would the manual focus be affected?

No. The problem is the XT kind of sucks for manual focus (I know. I used to have one and I used it with manual lenses a lot, so don't let that stop you). The viewfinder is small and relatively dark and the focus screen has no MF aids, so sometimes it's hard to see if you've achieved exact focus. And you'll find that the manual focus ring on the 50/1.8 kinda sucks for precision. The 85/1.8's focus ring will be better for this.

The best workaround is also one the XT can't do: liveview with 10x magnification can really help you nail focus precisely if you're working on a tripod.

But I think this may just be a matter of practice. It takes a while to learn how to autofocus with a fast lens wide open. I thought I'd had it nailed with my 85/1.8 and 50/1.8, and then I got the 135/2 and a manual focus 50/1.2 and I had to learn all over again. If you really can't get anything in focus, then you can test your lens/camera combination to see.

But given that you're saying it works some of the time, I think user error is the most likely culprit. Another thing you can do, if you've changed the setting, is to turn on the "beep" that happens with AF confirmation (like the AF point lighting up red). If you're hearing more than one, then you need to work on the finger press or do the back-button autofocus thing.

[carlyfan]

Originally Posted by inkista

Actually, not necessarily. All lenses and camera bodies are made to a tolerance range (say ±3). [These are made-up numbers, btw:] So, say your body is -2, and the 85 is +3, both are within tolerance, but your total out of whackness is 5 and you have issues. But if your 50/1.8 is -1, then the combo's total out of whackness is only 1 and so it works fine together. Kind of like that. If you send the 85 and your XT in to be calibrated, they'll both be set to 0. It may mess up some of your other lenses, which is why some folks recommend sending everything in all at once. Or buying a camera with autofocus microadjust so you can adjust yourself.

Ahhh, that totally make sense! Thank you!

Originally Posted by inkista

No. The problem is the XT kind of sucks for manual focus (I know. I used to have one and I used it with manual lenses a lot, so don't let that stop you). The viewfinder is small and relatively dark and the focus screen has no MF aids, so sometimes it's hard to see if you've achieved exact focus. And you'll find that the manual focus ring on the 50/1.8 kinda sucks for precision. The 85/1.8's focus ring will be better for this.

I'm actually kind of glad to hear that. I'm terrible at manual focus unless I'm working on a tripod with a stationary subject. I'm glad that it's better on other cameras. Maybe there's still hope for me!

Originally Posted by inkista

Another thing you can do, if you've changed the setting, is to turn on the "beep" that happens with AF confirmation (like the AF point lighting up red). If you're hearing more than one, then you need to work on the finger press or do the back-button autofocus thing.

I do have the beep on and it is helpful. I might have to try that * button-focus lock thing though.

Thank you so much for all your help inkista! I really appreciate it!

Lisa