AF Servo question.

[Simon Wantling]

Hi, it's been recommended to use AF servo for bird photography as this accounts for movement in the bird or the photography. I use a canon 7D with a 100-400 lens and what I've noticed is that the lens hunts on focusing with the result that a lot of the images are out of focus, even if I do the same on a tripod. Is this always something that happens with this focus drive and I might hit one out of five photos perfectly or is there something obvious I'm doing wrong.

Any advice would be great.

Thanks

Simon

[Roger Clark]

Simon,
Are you using a single AF point or all selected? If all AF points, sometimes there is conflicting info on the subject, and changing foreground, and background subjects that compete for best focus. Similarly, if the subject is small in the frame so the active AF area also includes foreground/background signals, then it can lock onto those. The active AF area is significantly larger than the AF rectangle in the viewfinder, and has fuzzy boundaries.

Roger

[Simon Wantling]

Simon,
Are you using a single AF point or all selected? If all AF points, sometimes there is conflicting info on the subject, and changing foreground, and background subjects that compete for best focus. Similarly, if the subject is small in the frame so the active AF area also includes foreground/background signals, then it can lock onto those. The active AF area is significantly larger than the AF rectangle in the viewfinder, and has fuzzy boundaries.

Roger

Thanks Roger. Yes, single AF point is selected. I think you may be right with the subject being small in the frame. I was photography a goldfinch from about 7 - 8 meters away, so this might explain it. It wasn't hunting a lot but you can just hear the lens altering constantly.

[Daniel Cadieux]

From that distance that is normal. The lens is likely picking up the subject and some branches and subtly alternating between them, especially when handholding as evrey small movement either sideways or back and forth gets the lens correcting focus. Try locking focus when you are happy with it.

[Simon Wantling]

Thanks Daniel, ok I don't want to seem dim but, how do you lock focus on the 7D? That's a new one on me and may be where I've been going wrong. Sorry if that seems like a stupid question.

Thanks

[Simon Wantling]

It's ok Daniel, I've changed the AF-on button on the rear of the camera to be AF-stop. Which means I think that once I'm happy with focus I can hold that button down to lock focus. I'll try that as soon as it stops raining here in the uk which is never at the moment.

Thanks

[Roy Churchill]

Simon, if you use back button focusing, say the AF-ON button then all you have to do is to press the AF-ON button to focus and then simply release your thumb from the button to lock the focus - much easier than what you are trying to do !!!