AF with oncoming subjects

<p>Hi<br>

Using a D3s with a 70-200 AFS VRII, I was out shooting kids sledding yesterday.. White snow, colourful snowsuits, should be pretty easy for the AF I figured..<br>

Using AF-C, with 9 points active, the camera and lens were quite capable of quickly finding my subject and locking in.. With the AF-ON button firmly pressed, as the kid would sled towards me, the subject stayed in focus until about the 7m range and closing. From there, the image got softer and softer as the subject got closer and closer. It's as if it lost the lock. Yet as my subject got closer, the frame fills up and I would think it would be easier for the camera to focus.. No ?<br>

The lens was set to focus on "Full", that is I didn't have it limited to 5m..<br>

Any ideas on what I am doing wrong ? Or is there some AF setting that is best for this type of shooting ? I have the same issue when shooting bike racing. It locks in, the AF is fine, as the rider gets closer, the image gets softer and softer...<br>

Any ideas ?<br>

Thanks!<br>

J</p>

<p>Also...even if the focusing was perfect? ... you may see as the subject moves in closer, the angular velocity will increase, and possibly you would need a much faster shutter speed to make sharper pictures.</p>

<p>You could test both the auto focus performance, as well as adequate for the movement shutter speed, by increasing ISO (just for the test), to use very fast shutter speed and make sure that the softness is not caused by the shutter being too slow for the fast action at close distance. Start testing with a single fixed central focusing point and make sure it stays on the subject, ...then increase the number of active points.</p>

<p>You do not say what Shutter speed and ISO you are using...</p>

<p>Also.. lens DOF changes with focusing distance, and it gets more critical at close focus.<br /> ... so you have few factors to test at various Aperture settings as well.</p>

<p>...see advice here:<br>

<a href="../nikon-camera-forum/00bD8X?unified_p=1">http://www.photo.net/nikon-camera-forum/00bD8X?unified_p=1</a></p>

 

<p>What Frank said is correct, but Nikon also is known for having trouble tracking subjects coming head-on. Even with the more advanced AF systems, like the D4 and D700, they struggle with it. It's an acknowledged design flaw in their AF system. I noticed it with my D700 while shooting track (hurdles and sprints) and did some research to find out why. Basically I got a response telling me to track side to side across the track from the infield rather than position myself at the finish. Only other way to do it, is to shoot "NASCAR style." Pick a predetermined spot on the track and AF on it, then shut off the AF and fire on burst until the subject crosses that plane. </p>

<p>This can be fixed fairly easily. Get a Canon.</p>

<p>Also, you may want to try the "AI Servo" shooting mode. The AI Servo adjusts the focus for moving objects.</p>