Sigma 70-200 mm lens 'hunting'

[MrShane]

Hi there, I am using a 70-200mm 2.8 EX Sigma Lens with a Canon 10D. When I push the button halfway to focus, the lens is continuously 'hunting'. Does this suggest it is not focussing properly? Some photos come out ok, others seem slightly off. If I take a sequence of shots perhaps only one or two are in proper focus.

I'd appreciate any feedback!

Thanks, Shane

[Clemmie]

If it's hunting, it's having trouble getting a 'lock on' - and this is common with AF on longer lenses.

With a multi-sensor camera, it is often helpful in this situation, to set the camera to concentrate its efforts on the center crosshair sensor - which is the only one that has a 'straight shot' to the subject, regardless of focal length. The signals for the outer sensors get 'lost' in the outer edge distortions of the lens, confusing the processor and causing the 'hunting'.

Using just the centerpoint for focusing can be a little tricky at first, if your subject is off-center. With a little practice, though, you can learn to get focus lock with the center sensor - then hit the focus hold, recompose, and shoot, within a fraction of a second.

[MrShane]

Hi there, thanks for that..will try out that focussing and see how I get on. Ta!

[paulnj]

ALSO....AF works via CONTRAST. If you have low light, you will HUNT

[Clemmie]

Generally True - which is why most newer cameras have IR assist for the autofocus.

Such as with my Maxxum 7, which is capable of focusing on black in the dark. Not every time, of course - but is quite surprising what it will do.