problem with targets moving at me

When I am on the slopes I have noticed a problem with my focus. I will have someone targeted and in focus and as i am shooting them the AF will focus all the way out and then come back in. So am getting sequences of people with a blurry middle shot. Sometimes I will be tracking them only to have the focus go out right as I take the picture. I have only noticed this as people are riding past or at me. It seems like the AF doesn't like them moving. I have had this happen in both One Shot and AI Servo. I have more examples than I would like to have and I will upload some. I am shooting these on a 50D with a 24-105L in AV mode most of the time wide open. any ideas?

Well first and foremost..............​IF your shooting any thing MOVING......you need to be in AI Servo

Lets think this through on "one shot"
One shot is for stationary subjects.....once you push your shutter button down half way, it engages the AF system (locks distance to subject)....so if your subject moves.....focus distance is still the same and out of focus shots every time.

AI servo is when you first acquire focus on your subject, and maintain your subject on your focus point, while maintaining pressure on your shutter button, the camera will continuously maintain focus up to the point of taking the picture.

In looking at your images.....its hard to tell if focused or not (too small to see detail)....so i will assume they are what you say. Based on the samples....AF systems struggle to focus when the color or contrast of the subject is dark. So in this case (possible point #1) the black jacket may have worked against you.

(point #2)
I would also suggest using the software that came with your camera (DPP) and using it to determine WHERE your focal point was.........really great tool when having focus issues. You may not have maintained the focal point on the skier (and if the images are cropped...will distort where you were aiming) and locked on something in the background at the time of capture.
This leads to if you are using focal point expansion.....while you need to be using center point for focusing on moving targets...but if you limit the camera to only center...it may cost you images if your technique is soft.

(point #3)
technique...........ma​ny issues here......but one that comes to the surface quickly is are you releasing the shutter button between shots??? if so, focus lag time may be the problem

hope this helps you in some way..........

Sometimes autofocus misses, regardless of body or lens. The most difficult autofocus task is to hold focus on a rapidly approaching target, especially with a busy background of the kind in the above images. Some camera and lens combinations are better than others, and the 1D Mark 4 has a good reputation for improved autofocus. It's better, but nothing's perfect.

i've had good luck with my 1d mk II by putting it in AV Servo mode, and then setting point to the middle (instead of auto select) and making sure i have that middle dot on my target.

Sick puppy thanks for your advice. I was on AI Servo for these shots. I was also not releasing the shutter. I was on low speed continuous.

I think your second point is most likely the cause. When I looked in DPP it showed the center focus on the guys hand in the blurry shot, so I might have been leading him too much and sent the focus to the trees in the back ground.

When you are not on a single point the camera wiLl pick the point for you. So in the second series that is what happened.

canonnoob wrote in post #9533549
When you are not on a single point the camera wiLl pick the point for you. So in the second series that is what happened.

I don't get it. In that series the focus is no where close to any of the lit up points. It is like between the bottom of the photo and the lowest AF point. Will it choose a point that is not inside the football AF shape?

Snowshooters wrote in post #9534742
I don't get it. In that series the focus is no where close to any of the lit up points. It is like between the bottom of the photo and the lowest AF point. Will it choose a point that is not inside the football AF shape?

those shapes are just guides... the AF does not end there... but how do you hold your gear when shooting? IT is possible that you bumped the focus ring...

I am usually holding the zoom ring which is on the inside next to the body. The focus ring is on the outside on the 24-105. There is no way I am doing it. I am shooting and its locked on and all of a sudden it zooms all the way out then back in to get focus again. It is really fast.

When you use all AF points, the AF will try to focus on whatever focus point that falls on the closest object to you. The AF likely picked up on the snow in between you and your subject but then lost focus due to the fact that the snow has very little contrast to for it to lock onto.

Never use all AF points, except maybe for birds on a clear day.

In the first one, you just didn't track perfectly and your AF point fell off of your subject and picked up on the background.

On way to improve your tracking is to shoot as tight as possible. Your AF point is less likely to fall off of your subject if your subject is filling your frame.

You have no idea how stoked I am to find out its just user error. I can fix that. I was worried something was broken. Thanks everyone.