Monday, September 28, 2009

Using Auto Focus: Part I, Moving Subjects

Great Blue Heron in Flight
One of the hardest things for someone new to a DSLR, or compact digital camera, with more than one type of auto focus modes is how to use the different modes, and in what situations. Each camera body is a little different when it comes to auto focus, another factor with SLRs and compacts with interchangeable lenses is the lens used. For example, when I was first getting serious about bird photography I used a Nikon 70-210mm AF lens, which auto focused using the motor in the camera body to drive the lens focusing mechanism. There are advantages to this method, at least in bodies that can house a powerful auto focus motor, like Nikon's D1/D2/D3, or D200/D300, is that the motor can focus even glass heavy lenses such as the 80-200mm F2.8D ED very quickly. The disadvantage of such a system is that on lower end consumer bodies like the D50, D70, D80 and D90 might not be as fast. Now, we come to the big disadvantage of the 70-210mm AF, auto focus speed, which is limited by the motor. Due to the zoom size of this lens, and the consumer grade gears in the auto focus mechanism, the speed isn't fast enough for birds in flight, period. The lens does snap into focus relatively quickly, but in continuous focusing mode, which is needed for birds in flight, it tends to hunt and loose focus.

Then we come to the lens that I use for bird photography today, the Nikon 300mm F4 AF-S ED. The AF-S indicates a built in motor and it offers equal auto focus speed on all camera bodies. Not only does the 300mm F4 AF-S offer the built in motor, it also focusing faster and more importantly, quietly. In continuous focus mode the 300mm F4 AF-S has proven to be far less likely to miss focus and hunt. Part of the reason behind that is the focus limiter, which means the lens focuses on a narrower scale when the focus limiter switch is set to 3m to infinity. The non-AF-S version of that lens also had a focus limiter, and it also is said to help AF speed, but it is limited to the high end consumer and pro cameras due to the need for the focus motor in the body.

Now I need to get back on track, auto focus modes. I talked about continuous auto focus, and now I'll give you a better idea of when it is best to use that. As I noted each camera body is a little different, so that usefulness of that changes as well. From now on I will refer to this AF-C, as it is called on a Nikon camera. For stationary subjects AF-C gives you the ability to move and not loose focus, although I would recommend not using AF-C for stationary subjects. Where AF-C becomes more important is moving subjects. When you are using AF-C with subjects that move in unpredictable ways I suggest using dynamic focusing mode, rather than single point (AF-Single). If you are focusing on a subject that is predictable, and that you can keep the focus point on, go ahead and use AF-Single focus mode. For birds in flight I always use AF-C in dynamic mode, because you can never be sure if you can keep the selected focus point on the subject. I also recommend using the continuous shooting drive mode, which means that as long as you keep the shutter button pressed down the camera will continue to take photos. Well that is about enough for today, I'll post more thoughts on this in an upcoming entry.