Focus issues today: Canon 50D and 17-55/2.8

eoren1

Ran into a strange situation today. Was shooting my daughter at a playground. She was backlit with the sun (about 3pm) fairly high behind her. Was shooting with the Canon EF-S 17-55/2.8 IS at 4.0 using upper focus point (in portrait orientation) and with AF set to AI servo.

The lens was all over the place as far as focus - hunting, nowhere near in focus (looked like pure bokeh) in some shots. I have used this setup countless times without difficulty. I've shot with the sun much lower without an issue as well. I changed to One Shot focus which was a bit better but still was a bit slow and felt it was hunting a little.

The lens/body have not been hit/damaged.
Any thoughts as to what could have caused this?
Thanks,
E

ziggy53

What was your daughter's activity at the time?

Did you have a filter installed?

Were any images captured with One-Shot? Were they in focus?

If you remove the lens and raise the mirror, do you see anything (hair, etc.) over the AF sensor?

Does AF continue to work normally since?

eoren1

Climbing wooden stairs on a playground slide thing
No filters
Yes lens hood
Sun not in frame
Switched to one shot with more in focus but, as i said, still felt a bit slow to focus and had some hunting
AI servo worked everywhere else on the playground...repeatedly failed in that spot. She wore white/pink again wood and green so contrast should not have been an issue.
Nothing on the sensor or mirror
Was quite frustrating...will have to keep testing the combo out and compare to my 70-200/4 IS and see what happens
Thanks for the post
E

ziggy53

While I don't think you should have had problems, based on your description of the scene, it does seem like something must have confused the AF sensor.

insanefred

have you just tried the center focus point just to see if it worked?

NeilL

The little focus point indicator in the vf does not show the circle of area which is being sampled to calculate focus. So, though you have the focus point where you want it, if the subject is in a complex setting, with many objects close to the subject at varying distances to the camera, the AF might calculate for something not your subject, more likely if your subject is moving and the proportion of your subject in the AF calculation field is sometimes less than larger/ brighter/closer/more contrasty objects.

I'd say you might find that that particular shooting scene which was giving you trouble has many high contrast edges, at different focal distances, occupying the same AF calculation field of the focus point you have chosen, as your subject.

Neil