Is my lens having focus issues, or is this normal?

[jorgemonkey]

I wasn't sure whether to post this here or in the Help Files forum.

My Sigma 70-200 is having problems focusing on objects coming towards it. I first really noticed it earlier this month at a bike race, and I payed attention to it today.

I'm on the end of a straightaway, with the riders heading towards me. Camera is set to continuous AF, and it will lock onto the rider. It will lock on for about a frame or two, then it quits focusing. Getting panning shots it works perfect, and when the rider is at an angle coming at me/going away it tracks pretty good.

Is my lens having this issues or do other lenses (aka the Nikon 70-200) have this problem as well? I never really noticed it before, so it could just be the lens needs an overhaul, or its always done this and I've never noticed.

I'll try to post a few examples today of a burst mode focus/OOF group.

[JSPhoto]

Are you continually focusing? I don't know how the Nikons track, but as long as I continually focus while tracking it changes the focus as the target gets closer or farther away.
The Canon bodies have various settings that can be adjusted as well as AF focus speed.
JS

[jorgemonkey]

Yeap, the camera is set so as long as the AF-ON button is held down (I have AF set for it instead of the shutter) it will continuously focus. I tried a few different AF modes today and they didn't seem to work.

I got a chance to look at my photos from the bike race today, and I'm very glad I wasn't being paid to shoot it. Out of about 1000 photos, I have about 300 keepers, another 75-100 of nice stuff, and the rest are OOF. Judging by the shots some have some back focus on them, others just arent in focus. I then looked at the photos from my 28-70, and about 85-90% are in focus, most of the OOF ones are user error.

Looks like my lens is going to take a trip back to Sigma for a checkup . I'm also gonna try to rent the Nikon 70-200 F2.8 for a bike race and see how it compares AF wise to the Sigma.

[specradio]

All of my lenses focus roughly when things are coming towards me.

[rovowen]

I have a Sigma 50-150 and was shooting long distance runners at the third turn. I found that when a runner was about 20 feet away the lens didn't focus. Farther away running toward me it worked as when they were running away. But seemed that at the 20 foot mark it just wouldn't focus when zoomed out near the 150mm.
Maybe this will help. Don't know if you were having the same problem.

[livin4lax09]

well in reality it is much more strenuous on the AF when a subject is coming towards you instead of away. So it makes sense when you have problems with stuff coming towards you. But from what you are saying it sounds like your subject is moving at a constant speed, which shouldn't throw the AF off that much. AI Servo (or follow focus) is predictive, so it will guess what the subject's next move is using a series of algorithms, but if you have a subject that is accelerating at a varied rate, then it is much harder to attain a lock. 0 acceleration is the easiest to AF, followed by steady acceleration, then mixed acceleration. So it doesnt sound like you should be having that much trouble with what you are doing. Maybe take the lens into a camera store and have them check it out for you?

[jorgemonkey]

I've asked the same question on another forum as well that I know has some cycling photogs, and I've gotten good advice. Some of the things I've learned so far and need to try out:

Try shooting slower than 8 FPS
Set my AF priority to Focus instead of FPS.

I'm sending the lens in anyway since I've abused (had) the lens for about 2 years, and I can hear some dust in there when I'm zooming, and its possibly gotten a slight backfocus problem now.

The test I plan on doing is I'm going to have a friend ride his bike towards me on a turn, and I'll try shooting him at different FPS rates and focus points to see if that makes a difference. While my lens is gone I'm possibly going to rent the Nikon 70-200 F2.8 and go shooting with it to see if there is any difference.

[Photo-John]

I'll second what Brent said. Using continuous auto focus while a subject is coming towards you is the most demanding thing you can ask of an AF system. My guess is that the AF system chokes and the lens just stops focusing. With my Canon lenses I'll just get out-of-focus photos when it gets to be too much for the AF.

Think about this - the closer your subject gets to you, the faster the relative speed. The AF system does have limits and it will eventually fail to keep up. Different lenses and systems will no doubt react differently. Longer lenses will work better as the relative speed is easier to handle. A 70-200 for bicycles accellerating at you on a straightaway is going to have a very narrow window of opportunity for predictive AF to work. Better to plan your shot and pre-focus. Or get a longer lens. A 300 or 400 will deliver better results under these conditions.

[jorgemonkey]

Ohh, I would absolutely love to get the 300 - My goal is to get it next year. I'm hoping to do some shooting for West Valley this semester, and when I get to a point where I can start selling more bike race photos that money will goto getting that lens. Right now I just can't justify buying it

I finally got the nerve to edit the photos from the race, and I must say the ones that came out sharp came out sharp. After hearing different replies from everyone it does sound like I'm maxing out the AF capabilities of the camera/lens combo. I was stoked about the fact that the lens still works well, and knowing better what it can/can't do will help me improve my technique better.

I'll get around to posting some photos from it during lunch today. I don't have enough time to post 'em during work