HELP requested -- AI servo focus problem

please help me figure out what's going wrong with these shots, and how to fix them.

The problem: I am in AI-servo mode, tracking the skater in the center of the frame. The camera sometimes seems to decide to focus instead on one of the other (non-center) focus points.

Setup: 20D, with 135 f2 lens. Same problem with my sigma 70-200, and same problem with a friend's borrowed EF 100mm f2. The camera is in M mode, custom white balance, the exposures are 1/500, f2.2, ISO1600. Shooting large JPG, parameter 2.

Some observations: I've only had this problem when shooting fast-moving objects in low light (hockey) -- doesn't happen with baseball, soccer, bicycles, etc... There is one corner of the ice with a few lights burned out and the problem happens more there. I have now recreated this problem using both AI-servo and AI-focus. The problem occurs equally whether I'm using my usual custom function 4-1 (focus with *) or 4-0 (focus with shutter button). I have also taken off my UV filter for a few shots without fixing the problem. Also, I think it's dependent on the amount of contrast in the background -- with hockey the backboards are close and there are nails, joints and other contrasty things to grab the camera's focus. (compare to basketball)

Some promises: I have really and truly set the camera to the center AF point, and double-checked it. I really was tracking the subject in the center point the whole time.

This is similar to complaints of the camera focusing on chain link fences at baseball games.

Here are some shots. All are full-frame, no additional sharpening, I've superimposed the 20D focus points (taken from the on-line 20D manual) and put in some blowups of the relevant focus points. One of the shots shows how my photos look when the skaters are not moving so fast, the others show the problem.

Thanks in advance for your attention....

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here are two more....

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I use single point focusing only it seems to help alot.
If you use all the points it seems to focus else where from time to time.

http://www.sportsshoot​er.com/members.html?id​=4723

I've taken about 25,000 shots of my 20D since may, I found at first I was having the same issues, now I maybe mess up on a few out of a 100 now days.

Luckee -- thanks, but I am using single point (center).


I select the center AF point using the multi-controller (joystick) with Custom function 13 set to 1. I know I've picked the center AF point, as I see it light up when I press * to focus.

I keep the center point squarely on the logo in the center of the skater's jersey. The camera seems to be "handing off" focus to one of the other focus points, even though it's not selected.


Here's a quote from the 20D manual on the subject of predictive AF in AI-servo mode -- "When the AF point selection is automatic, the camera first uses the center AF to focus. During autofocusing, if the subject moves away from the center AF point, focus tracking continues as long as the subject is covered by another AF point. ... With a manually selected AF point, the selected AF point will focus track the subject"

So the problem as I see it is that the camera isn't supposed to use the peripheral AF points when I am in AI-servo with the center point selected, but it seems to be using them anyways, and switching focus to them when the main subject is fast-moving and low-light.

So the problem as I see it is that the camera isn't supposed to use the peripheral AF points when I am in AI-servo with the center point selected, but it seems to be using them anyways, and switching focus to them when the main subject is fast-moving and low-light........hmmmmmmmm​mmmmm I know i had this problem before just cant seem to think why thats doing it still. I know the more I was locking as the subject moved I seemed to capture better focused shots.

Have you tried the other AI Servo tracking sensitivity settings -- I assume the 20D has options. It's custom function 20 on the 1DmkII.

Does the 20D have CF-17 for focus point expansion? If it does set it to 0.

Unfortunately not. CF17 on the 20D is the Lens AF Stop Button functions for those rather expensive L primes.

I can't see anything remotely like it in the other Custom Functions either.

Plotting the focus points over the pic of a moving subject doesn't help. The camera has to focus *predictively*. What matters is how well you tracked the moving subject with the center focus point before you pressed the shutter button. After you press, about 65 msec goes by - a lifetime in the world of predictive focus - and your suject has moved a couple of feet. The predictive focus algorithm has to compute where the subject will be when the shutter actually opens and closes.

Maintaining a steady position on a moving subject is much harder than one realizes. Your skater is actually occupying a relatively small portion of the frame; and you're using ~ 200 mm lens (135 X crop factor) which of course magnifies any unsteadiness. It takes a lot of practice to keep your subject perfectly in the center of the frame.

You'll note the most good action shots typically have subjects that nearly fill the frame. As you demonstrate, your camera tends to focus on the back rink wall - which fills more of the frame than the targeted skater.

fslshooter -- 20d doesn't have servo tracking sensitivity settings. could you please explain to me what these do on 1-series cameras? in other words, would they solve the problem?

Ken -- sadly, GSH is right that there's no AF point expansion setting, or anything like. again, what exactly is this and would it solve the problem?

all, thanks for your attention. this problem is driving me nuts, and I appreciate the help.

Woody -- not sure what you're suggesting here. the last of the images posted above (solitary skater) was actually after the game, I asked him to skate smoothly around the corner circle while I practiced tracking and so for that shot at least I'm pretty sure the motion was uniform and I was tracking the logo on the jersey as carefully as I could. Thanks for looking

I'm suggesting the camera is not actually tracking what you think. Take and review a burst sequence of 5-10 shots and evaluate them all. See if the camera is doing what you think it is suppose to do.

Also, compare tracking a skater laterally with tracking one going away or towards you. The each pose different challenges for you and the camera.

I frustrated myself with exactly this same scenario - a hockey rink and a 20D. Sometimes the problem was me; sometimes the camera didn't seem up to the task. Figuring out which problem explained poor focus on a given shot was very tough.

Woody --

in a burst, perhaps the first couple are in focus and then it shifts to back-focus as above, without the center focus point leaving the subject.

problem only occurs when there's a component along line of sight - though it can be diagonal motion as well. tracking lateral motion (in-plane, or perpendicular to line of sight) doesn't seem to cause the problem - Nor does it ever happen with a still subject.

frustrated is the right word, the worst part is that I can't see what's causing it...

thx again,

In addition to the default, there are four settings for servo tracking sensitivity. Adam Christopher's Magic Lantern Guide probably explains the function and settings best. I quote: "If a subject moves among other subjects (such as a running back in a football game) or other things move near the subject, focus can be thrown off. This setting lets you change the camera's sensitivity to other objects (but it doesn't change focus speed).
0. Default. Standard factory set sensitivity.
1. Slow
2. Moderatley slow
These (settings 1 and 2 ) cause the camera to react slowly to objects passing between you and the subject, possibly keeping the subject in focus longer.
3. Moderatley fast
4. Fast
These (settings 3 and 4) make the camera see new objects quickly as they come between you and your original subject."

CF 20-2 seems to work best for me for shooting baseball. I find Woody's reply very interesting and it might explain why I haven't been able to nail a shot of a runner rounding second base when I'm shooting from the third base line. I want to capture the runner immediately after he rounds second and turns toward me while he's leaning into his turn. I track him laterally as he runs from first to second but when I fire he's running toward me and perhaps the predictive focus algorithm that Woody speaks of expects lateral vs vertical movement. I have found that if I shoot when the runner is half way between second and third or sliding into third that the focus is much better.