5 May 2008
Improving Autofocus Performance - TWIP
Posted by Scott under: Tips .
Photo by Scott Bourne
Most Digital Single Lens Reflex (DSLR) cameras use a passive autofocus system. That means that computer chips in the camera analyze how far away the subject is from the sensor plane.
These systems rely heavily on contrast. That’s why when you have low light, low contrast scenes, autofocus rarely works well.
If you want to get the best autofocus possible on your digital camera, aim at an area of high contrast on your subject. For instance, if you are photographing a blonde person against a black background, point the autofocus sensor at the area where the hair and the background merge to get a focus point. Then allow enough depth of field with the proper aperture selection to get the face features in focus.
Light has a huge role in this play. Passive autofocus must have light and image contrast in order to do its job. So you can improve autofocus by using flash or by adding light via reflector or other methods.
Here’s another tip. Your autofocus system usually reacts better to vertical rather than horizontal detail. That means that you will have a better chance of getting a flag poll in focus than a ship on the horizon. Newer and more expensive cameras have solved this problem by using two autofocus motors, one that works on the vertical plane and the other on the horizontal. But you can still improve your chances of a sharp image if you concentrate on something vertical in each scene.
If your camera has selectable focus points, you will always get faster focus by pre-selecting the point that is on your main subject. Also note that most DSLRs have designated the center focus point as the strongest. So it might make sense to move the center point over the subject, press your camera’s focus lock button and then recompose the image to make the picture.
Finally, most cameras autofocus by pressing the shutter button halfway. But sometimes, in the heat of the moment, we forget to pause at the halfway point and let the camera acquire autofocus. Try to get in the habit of pausing for a moment before you depress the shutter button. This will give the autofocus system adequate time to do its job and also offers the side benefit of reducing camera shake.
Of course those of you lucky enough to have high-end cameras may have few of these problems. It’s still good information to know.
6 Comments so far...
Graeme Smith Says:
5 May 2008 at 7:58 am.
Actually not all AF systems work better with vertical rather than horizontal detail. On my Nikon D40x the center point is a cross type sensor (works with both vertical and horizontal detail) while the two edge sensors are horizontal detail only (assuming you have the camera in horizontal orientation).
” by using two autofocus motors”
That should be two autofocus sensors. Do you need an editor, Scott? 
Bruce Elliott Says:
5 May 2008 at 8:03 am.
Make sure if you have a camera that has the choice to set your autofocus to ‘S’ for a stationary subject or ‘C’ for a moving subject. Those are the Nikon settings, not sure if Canon is different. If you’ve been shooting moving subjects with C and forget to switch back when you’re following the advice in the above post you’ll find that when you autofocus with the centre focus point and recompose the shot holding the shutter halfway down the cam won’t hold the focus point so you’ll end up with a blurry shot. Always check ALL of your settings before going out to shoot!
Scott Says:
5 May 2008 at 8:40 am.
Graeme actually all the AF systems I know of do in fact work better that way - but because I know there are folks out there looking to critique my every word - I used the word “usually” as a qualifier. So sorry you don’t get your gotcha moment there. And if you have actual experience that differs - not something you read about in a forum - then feel free to share it.
And no I don’t need an editor. Most (there you go again - I said MOST) cameras like those made by Canon - use an actual motor to achieve the autofocus - it’s called USM on Canon and the motor resides in the lens. These work in conjunction with the sensor. Thanks for playing.
Chris Bricker Says:
5 May 2008 at 12:16 pm.
Can some one explain the AF schemes that are in cameras? I have the Canon XTi Rebel and I have “One Shot”, “AI Focus”, “AI Servo”. Can some one explain the difference between these three. And maybe how the camera interacts with these schemes.
CTDL 169: DSL Autofocus on TWIP Says:
7 May 2008 at 3:59 am.
[…] [From TWIP ยป Improving Autofocus Performance - TWIP] […]
Tom Dibble Says:
8 May 2008 at 12:57 am.
“Newer and more expensive cameras have solved this problem by using two autofocus motors, one that works on the vertical plane and the other on the horizontal.”
I know you addressed this before, but it still appears incorrect. Are there multiple motors in the lens? And, if so, how would one work on the horizontal [detail] and the other on the vertical?
One tip I have is for relatively stagnant DOF in a situation with autofocus “hazards”, such as a chicken wire fence ~2 inches away from the lens while you are shooting animals 50 feet away. To avoid the autofocus seek penalty box (maybe not a serious problem if you have super-expensive fast-motor lenses, but it always aggravates me), get the best autofocus you can on the distant subject, then *turn off autofocus* (usually via a physical switch on the side of the lens, at least for Canon mounts).
IMHO, the “ideal” would be that we could somehow tell the AF system “don’t even try focussing on something closer than [x] inches” … would make shooting softball games (often through chickenwire) where the above trick won’t work, much easier. Doesn’t seem like this is an option out there though.
While I’m on wish list items, I also wish there was some way (aside from thunking down into manual focus) to tell the camera “focus two inches in front of what’s there”. Shooting a softball game, I know that most of the batters are going to lean forward (toward the camera) a few inches when they swing the bat; the perfect plane of focus when I have time to half-press the shutter is a few inches back from the perfect plane of focus when the bat hits the ball. Every single time.
Finally, about “continuous AF” or “AI Focus” (I believe) … I tend to not use these as I’m a religious focus-recompose guy, and these modes tend to continuously autofocus on the active AF point … when you recompose, now it’s wanting to AF on what’s *now* in the middle of the screen. Cycling through AF points is too much to deal with in the heat of a shoot (almost always sports shoots where I’d want to do this, and having to worry about which of the AF points is active when I bring the camera up to my face is a bit too much.
