Autofocus question

[grantyale]

This problem has had me for a long time. When trying to focus on a subject in front of backlit foliage (say, kid standing under a tree), in a wide-angle setting, the camera very often choose to focus on the background. I'm thinking perhaps it's the difference in contrast and AF point coverage that cause this problem.
Currently the workaround is to zoom in and focus on the human subject and zoom out to take the picture. I'm wondering whether there is a way to get rid of this problem totally.
I'm currently using a Sigma 18-50EX on a 400D. Thanks in advance.

[Anti-Social]

Is it autofocus or exposure? You can't move in lock the focus and move away cause the focus would go off. Did you select the af point correctly?

[Dream Merchant]

Try 'shifting' the central AF sensor (and select only the central one) a little over the most solid part of your subject that completely covers the central AF sensor area and tap your shutter a few times. Select one-shot AF mode or use AF lock. If there's any part of the sensor that 'sees' outside of the solid part of the kid, e.g., the edge of his head or shirt, and picks up the much higher contrast BG, chances are it will focus on the BG.

Another alternative is: Select and use only the central sensor, select one-shot AF and focus on the kid's feet away from the BG, or anything else along the same plane as the kid. Shift to re-compose your shot and fire. Since you're at a wide setting, you probably have a little more room for error in terms of DOF.

Then there's always manual focusing with audio and visual focus confirmation.

Hope this helps.

[Redsun]

select the focus point thats over your subject and half press the shutter to lock the focus
"zoom in and focus on the human subject and zoom out to take the picture." will not help you to focus on the subject correctly

[sk.images]

Originally Posted by grantyale Currently the workaround is to zoom in and focus on the human subject and zoom out to take the picture.

I would be surprised if you were getting sharp images with this method, unless you're using a very small aperture, and even then it would be luck. My guess is that if you subject is backlit then there is too little contrast on the subject, try focussing on the edge of the subject or just use manual focus.

There have been a very few zoom lenses made that have the capability to maintain focus while changing zoom, but they are quite rare and my guess very expensive (and probably not made by Canon).

[Dream Merchant]

Yeah, forgot about the 'holding the shutter half-way down' after getting focus sonfirm! LOL!

[Anti-Social]

Put it simply

1) Select the AF-box on the subject
2) Make sure the subject fills the box
3) If these two doesn't work then you got a backfocus problem, please send the camera or the lens for servicing. You can test it at your home for ex put 3 items in a row and focus on the middle with the smallest depth of field and see where the camera ends up focusing, on the item in front, behind or exactly in the center.

Pray it isn't backfocus or prepare to spend some $$

[grantyale]

Some clarification - The setup doesn't backfocus. The reason zoom-in, focus, zoom-out works is that 18-50EX is parfocal. I either keep the shutter button half-pressed or move AF-on to * button altogether. The composition doesn't allow any AF point to be filled by the subject - and that's why I zoomed in to fill the AF point (in fact the "box" is more or less filled but the AF sensor certainly extends way beyond that, so I had to make it more than filled).

[grantyale]

Erm...ok...I forgot to mention that center AF point is used at all times.

[Anti-Social]

Increase the depth of field, no choice but to get the boy in front of the tree to be sharp through depth of field. Nothing much you can do unless you do manual focus and take the picture, check then retake again until you get it right.

[grantyale]

So the conclusion is better stick to the zoom-in, focus, zoom-out with parfocal lenses...

Thanks to everyone.