My 70-200mm f4.0L - focus problem?

[BritishBeef]

I recently purchased this lens and went out to test it out. All seemed well until I took a series of images like this:

http://www.pbase.com/britishbeef/image/42022402/original
EXIF:
Shutter speed: 1/750 sec
Aperture: 4
Exposure mode: Av
Flash: Off
Metering mode: Center-weighted average
Drive mode: Single frame shooting
ISO: 100
Lens: 70 to 200mm
Focal length: 70mm
Subject distance: 3.88 m
AF mode: One-shot AF
Focus point: [- - O - -]

I was concerned that whilst focussing on the metal fence post in the center the AF beep kept indicating a lock while the viewfinder image showed clearly otherwise (as you can see in the resulting image link above. I tried many times and 8 times out of 10 the result was the same. Is this likely to be due to the object I was shooting being too hard to get a lock on? I've never experienced this with any of my other lenses.

The other shots I took seemed fine, here's an example:

http://www.pbase.com/britishbeef/image/42022793/original

Any help apreciated.

Regards,

Geoff.

[RichardtheSane]

Hi Geoff

Looking at the image the contrast betwen the fence and the background is not that great, and the AF system relies on good contrast in order to focus correctly. I can see from you other shots (the cat is wonderfully sharp :) ) that the lens is fine so I think it is the subject that causes the issue here.

[tim]

Do this test (http://photography-on-the.net/forum/showthread.php?t=66959) on a tripod, post that picture.

[BritishBeef]

Here's the test on a tripod, cable release and mirror lockup on 10D:

Focus point is on the '25'

Autofocus:
http://www.pbase.com/britishbeef/image/42026455/original

Manual focus:
http://www.pbase.com/britishbeef/image/42026456/original

Looks ok to me. what about everyone else?

Regards,

Geoff.

[JusSmith]

nothing wrong there!

[tim]

Looks good to me too.