Canon L lens with autofocus issues

[Tashique Alam]

i just bought my Canon 24-70mm 2.8 USM lens from singapore, and even though most of the pictures were tack sharp, about 25-30% of the images, specially the one taken in low light at the wider apertures ended up having severlely backfocused!!! i queried the issue over the net, and people suggested i confirm with using autofocus testing charts like these: http://focustestchart.com/focus21.pdf

true enough, after carrying out the test my pictures were actually coming backfocused

i have no idea what to do about it. there is no Canon repair center in my country, and my SLR the Canon 550D does not have any autofocus correction system.

is there any solution? did anyone else have this problem? can a lens be re-calibrated ?

i have given a sample of my test chart result.

[OsmosisStudios]

See: http://digital-photography-school.co...us-issues.html

It'll likely get moved. This thread is the newer of the two, though.

[OsmosisStudios]

These tests can be VERY unreliable if they're not done perfectly.

[Nicole]

Originally Posted by OsmosisStudios

See: http://digital-photography-school.co...us-issues.html

It'll likely get moved. This thread is the newer of the two, though.

Yep, merged and moved into the Canon forum too.

Tashique, I have no idea what the answer is to your question, but just a friendly reminder, please only post 1 thread with a question. It helps keep the answers in one place.

[inkista]

Don't test with a slanted ruler. Test by shooting a poster on a wall that you know you're perpendicular to. In good light. With a tripod. Ideally with some way of confirming you're exactly perpendicular to the wall, without having tilted or rotated the camera to be slightly askew. When you demand less than 2mm precision in your measurements, you better have that kind of precision in your testing methods.

Look at the LensAlign design to see what's really required for consistent, repeatable accuracy in this type of measurement.

Personally, I think your lens and camera are probably autofocusing just fine. You're just expecting too much of it, wide open, in dim light, with low-contrast targets. Try manually focusing under those conditions and then autofocusing and see which one's more accurate.