missing focus point on 1Ds2 - camera or lens?

[narikin]

I'm using a 1Ds2 with 85mm 1.2L II lens, on the street in good daylight, trying to get shallow depth of field to pick out figures from a crowd, but...  it seems to miss the focus point more often that not.
do I have a faulty lens or camera - or are my settings a bit wrong?

set up is:
single point AF, (no focus expansion)
AI servo AF
servo tracking: moderately fast
and I'm selecting one of the main central sensors (cross type) in the AF array.

what is wrong here?
the focus seems to usually be behind the subject, though not always...

do I need to return the camera body or the lens to Canon?

its a simple sports type shallow DOF image I want, but am failing 60% of the time so far, so any advice/tips/ experience gratefully received

[Geoff Wittig]

I have the 1Ds II with a lowly 85 mm f:1.8, and even with this the depth of field is generally so shallow that one eye will be out of focus unless the subject is square on to the camera.
I'd suggest running a simple test; line up something like 12 beer bottles in a row (hopefully not after drinking them all) and then focus on bottle number 6. This will tell you quickly if the problem is your equipment, or just the inherent miniscule depth of field and uncertainties of passive phase-contrast AF.

[gerry s]

I have the 1Ds II with a lowly 85 mm f:1.8, and even with this the depth of field is generally so shallow that one eye will be out of focus unless the subject is square on to the camera.
I'd suggest running a simple test; line up something like 12 beer bottles in a row (hopefully not after drinking them all) and then focus on bottle number 6. This will tell you quickly if the problem is your equipment, or just the inherent miniscule depth of field and uncertainties of passive phase-contrast AF.
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You could try putting tha camera on a tripod at home, auto focus on shallow object 4 feet away at 1.8, turn off af and move camera forward 6 inches, take another exposure, move 6 inches behind original position and take third exposure. open in ps and check exposure of all 3, should let you know if its back or forward focusing.

[Jonathan Wienke]

set up is:
single point AF, (no focus expansion)
AI servo AF
servo tracking: moderately fast
and I'm selecting one of the main central sensors (cross type) in the AF array.

You should use One Shot instead. AI servo is intended for tracking fast-moving action, and selecting one face out of a crowd is not what it is designed for. The motion-predictive algorithm in AI servo looks at multiple focus points even if you have a single point selected. The options you mention are only really valid in One Shot mode.

[narikin]

You should use One Shot instead. AI servo is intended for tracking fast-moving action, and selecting one face out of a crowd is not what it is designed for. The motion-predictive algorithm in AI servo looks at multiple focus points even if you have a single point selected. The options you mention are only really valid in One Shot mode.
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thanks, useful advice, I'll try it.

the faces/heads are of course moving, so I'd presumed AI servo was right, but will give single shot a go.