Your digital SLR camera is a bundle of some of the most advanced pieces of optical technology. It involves, metering, auto-focusing, image stabilization and a plethora of other refinements. In this article we are going to delve about auto-focusing.
Auto-focusing is a technique which allows a camera to lock focus on a point with the aim to effect the maximum contrast. Note – contrast and focus does have a relationship. When you press down the shutter button the camera does two things. It tries to adjust the position of some of the elements inside the lens to check if it produces a sharp contrast. This it does by using tiny phase detection sensors located at the back of the camera and the tiny auto-focusing motors that move the focusing elements into position.
On a typical DSLR there are three different types of auto-focusing. These are continuous auto-focusing, one-shot or single shot auto-focusing and automatic auto-focusing. Now, different manufacturers name these differently. Canon refers to them as One-shot AF, AI Servo AF, and AI Focus AF. Nikon prefers to call them Single-servo AF, Continuous-Servo AF and Auto-Servo AF. Regardless of what names the manufacturers put they tend to do things more or less the same way. Let’s learn a bit about these and find out when to use which.
Single shot auto-focusing is ideally suitable for locking focus on subjects that are standing still. Examples are buildings, a subject standing still and looking at you, the moon (though it’s moving but for the relative duration of the shot it is perceived as to be fixed) and so on. Single-shot auto-focusing is unsuitable for the purpose of photographing a moving subject.
99% of time I have single-shot auto-focusing activated. Why? Because I often employ the focus and recompose technique. In this technique you first need to lock focus on the subject that you wish to photograph by pressing down the shutter release button half-way. Once focus is locked you can then recompose while holding the shutter release down. You would ask why the focus and recompose technique, when you can select an AF point at the far corner of the frame? There are two reasons. A lot of older entry level cameras have only about 9 or 11 AF points. That kind of limits of the options. Second, the middle AF point is often the most sensitive, being a cross-type or dual-cross type AF point. Older entry level DSLRs don’t have more than one cross-type AF point.
Continuous auto-focusing, as the name suggests, keeps the subject in focus even though it is moving. This is achieved by moving the active AF point across the viewfinder as the subject is moving. To enable this you will need to select continuous auto-focusing and press down and hold the shutter release button half-way down while you are following the subject. Let’s say that you are following a cyclist as she drives down the street. As you pan holding the shutter button down half-way, you will notice that the active AF point switches from one point to the other on the viewfinder.
Auto-Servo basically takes the guess work out of focusing. When you set your camera auto-focusing to auto-servo you are basically handing over the decision making to the camera. The camera looks at the scene, determines whether the subject is moving or is stagnant at a point and then accordingly chooses the appropriate focusing mode.
Auto-Servo is ideally suitable in a situations where you cannot determine the correct auto-focusing mode. For example, a flower normally standing fixed can sway even in gentle breeze. Switching to continuous auto-focusing or single-shot auto-focusing may not be feasible. Switching to auto-servo allows the camera to switch instantaneously to the most appropriate auto-focusing mode depending on the situation.