Question: My daughter competes in bicycle and ski races so I often take photos with an EOS T2i and an AF 70-300mm f/4-5.6 zoom lens but many of them are not in sharp focus. When I shoot a series of nine photos of her racing toward me, maybe four will be sharp while the others will not quite be in focus. My brother has a Sigma 70-200mm f/2.8 HSM zoom and a lot more of his action photos are sharp.
Answer: Your camera has a very effective continuous (predictive) autofocus system that can track a moving subject effectively. For the best results, be sure to set it to the Servo AF option, not the AIA autofocus mode. The latter does activate continuous autofocus when the system detects motion but may not do so immediately while Servo AF provides full-time tracking focus.
If the action is moving very fast (downhill, for example), I also suggest activating only the single, central focus detection point. That will allow the camera to provide the fastest possible continuous autofocus but it will require you to center the primary subject and that works best when she is close to the camera.
When the subject is large in the frame, activate only the central focus detection point for the highest success ratio of sharply focused action photos when the camera-to-subject distance is changing rapidly. (Continuous Tracking AF; Nikon D700; 70-300mm f/4.5-5.6 G IF ED VR Silent Wave lens) (c) 2011 Peter K. Burian)
Granted, even then, some of your action photos may not be sharply focused, especially when made at 200mm to 300mm and especially on overcast days. That's because your lens' maximum aperture is quite small at long focal lengths; that minimizes the amount of light reaching the autofocus sensor. By comparison, an f/2.8 lens (with f/2.8 at all focal lengths) provides far greater light transmission for superior AF response. When possible, use the 70-190mm range with your lens for action photography on dark days because the aperture is still acceptably wide (f/4 or f/4.5) then.
Also, be sure to use a fast shutter speed such as 1/1000 sec. Often, blurring that's diagnosed as caused by a focusing problem is actually motion blur caused by a shutter speed that did not fully freeze the motion.
A wide aperture 70-200mm lens with ultrasonic AF and internal focusing (IF) provides fast, reliable Tracking AF. does provide faster, more reliable tracking AF than a 70-300mm f/4.-5.6 zoom with more conventional AF. Granted, its 201-300mm focal lengths are useful in action photography but a zoom lens of that type is not available with a constant f/2.8 or f/4 maximum aperture. (Nikon D700; 70-200mm f/2.8 G ED VR II Silent Wave IF lens) (c) 2011 Peter K. Burian
As well, your brother's lens is equipped with an ultrasonic focus motor (called HSM by Sigma) and internal focusing (IF) definite benefits over your zoom. If you're serious about action photography, consider buying a lens with ultrasonic AF such as the Sigma 70-200mm f/2.8 HSM or Canon's 70-200mm f/4L USM. (Longer focal lengths are very useful for races so you'd also want the matching 1.4x teleconverter or you might prefer a 300mm f/4 lens.) I have tested the recommended zooms when shooting go-kart races and equestrian events and can confirm that both provide superb image quality and very fast tracking autofocus.
- In addition, Peter is a contributor to two new books: The BetterPhoto Guide to Creative Digital Photography (just published) and The BetterPhoto Guide to Photographing Light (to be published in April 2012), both co-authored by Jim Miotke and Kerry Drager