Weird focusing issue

[xavier_xw]

Recently found a weird issue on the AF for my EF 85mm F/1.8 lens.

When using outdoors, the AF is working perfectly. But indoors, when the focusing distance is about 1m, the lens always back-focuses.

anybody has this issue before? thanks

[Deadpoet]

were you exceeding the minimum focusing distance, and therefore, in an attempt to press shutter, accidentally focus to somewhere behind the subject? Outdoor, you probably stepped back a bit, indoor no spce?

[Stratix]

if it's too dark, you probably need the AF assistance from your flash to get an accurate AF. =)

[xavier_xw]

I used a tripod, and the distance is 1m > 0.85m (min focusing distance)

[xavier_xw]

Originally Posted by Stratix if it's too dark, you probably need the AF assistance from your flash to get an accurate AF. =)

Yes, I tried that too, but still back-focusing

[Deadpoet]

a trip down to Harbor Front!

[Jemapela]

This is indeed weird, however not totally impossible. I have encountered such behaviour with other lenses at my work place.

Try this:

Shoot a brightly lit target that has high contrast (say black and white stripes) at 1m, and again the same target under same lighting conditions at (about) 4.2m. Use only f/1.8. Reset the distance scale to infinity before taking each shot. Press the shutter release button only once to allow the AF to lock-on -- don't press a few times to let AF to re-adjust itself. Take 5-6 shots at each distance and check to see if it has a tendency to front/back focus.

I dropped my 85mm f/1.8 and ever since it became persistent and consistent back focus. It's now semi-dismantled and I'm trying to fix it.

[xavier_xw]

Originally Posted by Deadpoet a trip down to Harbor Front!

forgot to mention this:

I did bring this lens to Canon service center, guess what? they said the lens is functioning perfectly! So I took several shots there and showed them the back-focusing issue. They couldn't provide any explanation but said again and again the lens is OK and the focusing will be precise outdoors (and it does so I just leave it that way)

[spazzer]

Originally Posted by xavier_xw forgot to mention this: I did bring this lens to Canon service center, guess what? they said the lens is functioning perfectly! So I took several shots there and showed them the back-focusing issue. They couldn't provide any explanation but said again and again the lens is OK and the focusing will be precise outdoors (and it does so I just leave it that way)

u muz insist that it is spoilt!

[xavier_xw]

Originally Posted by spazzer u muz insist that it is spoilt!

Yeah, but I tried in outdoors, it does fucoses perfectly most of the time, even at aperture wide open.

any other user care to take a test?

[Jemapela]

Try the test method I described, and describe your findings, that is, how back is it at 1m and at 4.2m.

Actually, I'm not surprised if you detect this phenomena with other Canon lenses.

[xavier_xw]

Originally Posted by Jemapela Try the test method I described, and describe your findings, that is, how back is it at 1m and at 4.2m. Actually, I'm not surprised if you detect this phenomena with other Canon lenses.

tried it, about 0.5cm back when at 1m, and acurate at ~4m

[xavier_xw]

do you mean this phenomena is common in canon lense? why is that?

[Jemapela]

I don't know if it's common but I have noticed it before.

However, do bear in mind that at near distances, as in your case at 1m, your depth of field is shallow, hence errors in the AF become more prominent. At further distances say 5m, the depth of field is deeper, hence errors in the AF is less prominent. I'm quite sure your less also back focuses at further distances but the back focus is less obvious due to depth of field.

Back focusing by 0.5cm sounds quite significant to me but Canon does allow tolerances in AF errors. I'm not sure if this is within the tolerance.

[xavier_xw]

thanks, I will do more test outdoors and see how it goes.

[Override2Zion]

Hi bro Xavier, regarding back focusing, perhaps you could perform a test.
Download the PDF document from the provided link and follow the instructions.
http://www.nikonware.com/files/FocusChart.pdf

This was initially created to test Nikon cameras and lenses for back/front focusing issues, however the procedure applies to all cameras. Perhaps you could give it a try to get a feel of how bad your back/front focusing is. Of course, the test results are just approximates, dun rely too much on it. Best to confirm that its the AF prob then bring your lens and camera to canon for servicing.

Hope this works for you.

[xavier_xw]

thanks a lot!