I recently received an email with some quesitons concerning autofocus. I thought the question and answer might make a good blog entry.
--- Hi, I'm an amateur photographer and have a Nikon D70s. I took one of your classes to help me feel more comfortable with using this camera. However, I've found that when I use the autofocus setting I have several pics that are not in focus. Is this a problem with the Nikon, or is it operator error? I have a Tamron 18-200 af aspherical xr di II lens on it. Could this be part of the problem? Any suggestions/help would be greatly appreciated. Thanks so much. ---
Autofocus has gotten very fast and accurate over the last decade, but still requires us (as photographers) to make some choices. Essentially it boils down to this: Single or Continuous. Single means that once the camera finds the focus for the active point, it locks that focus until after the frame is shot. That's good if you're photographing stationary objects, or doing portraits. Continuous means the focus will continue to adjust based on movement of the subject, which of course is good for sports (or photographing children!). This choice is sometimes made on the body, in the menus, or both.
Beyond that there are some other options as well. "Dynamic" means that if the active AF point moves off the subject, the system will look to the other AF points to see if one of them has now picked up the subject. If so, the system "hands off" the focus to the new point. "Closest Subject" means that the AF system will focus on the closest thing that a focus point is touching. Lastly, AF systems depend on contrast and light. The less you have of either, the harder it will be for the system to find focus. In other words if you try to focus on a gray sky, the AF system will likely get confused. If you try to focus in a dark room, it will also struggle (or shine a brief beam of light from the camera to help it out).
Bottom line is I doubt that the lens or camera is to blame. More likely one or a combination of the above options is at fault. I'd suggest you spend some more time with the manual and the Autofocus options, and play with the different choices to better understand them. The AF systems today are pretty "smart," but still need us to tell them what to do.
Best - Reed
