Lens doesn't always get focus spot on?

Donnie

Donnie

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Paul
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Im not sure what it is but I've noticed my sigma 24-70 f2.8 sometimes has difficulty autofocusing? instead of simply half press of a shutter and focus it will sometimes seem to hunt a bit.

This is in no one particular situation, ive deliberately tried it in good conditions, decent contrasting situations etc and not in low light either.

Im wondering if anyone has any thoughts on what may be causing it and if there's anything I can do ?
 
There are many variable to take into account here, the camera your using, the autofocus setting, continuous, single shot. Also the number of focus points, single 5, 11 ect. Also the subject is it contrasty brightly lit ect. How old is the lens, has it been dropped damaged? What apertures does it occur at f2.8 f15 or through the range. I own the Sigma 24-70 F2.8 EXDG and have found it reliable for autofocus, you can accidentaly slip it into manual focus mode by pulling the lens from its A setting to manual by removing from the camera bag, I have done this a few times.
 
seems to be all f stops single shot and using single focus point as I always do :-/ im the second owner and dont believe its been dropped or damaged, had it over a month now and only recently noticed the issue :-/

I'll have a specific testing day tomorrow I think and see if i can narrow it down at all.
 
I was having the same problem with my 70D and 300 f4L IS the other day. I was trying to focus on a BIF but the focus just wouldn't move. It was because of a lack of subject contrast on the focus point as I was pointing into a clear blue sky with just the bird in it. Once I'd manually focussed somewhere close it found it and locked on fine.
 
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