Different Auto-focus options
¨ Single point Spot AF (manual selection) For pinpoint focussing. Great for macro photography
¨ Single-point AF (manual selection) Select one of the available AF points for focus
¨ AF point expansion (manual selection) A manually selected AF point along with 4 adjacent AF points above, below, left and right. This gives more leeway in focus with moving subjects.
¨ AF point expansion (manual selection) A manually selected AF point along with the surrounding AF points are used to focus
¨ Large and automatic AF selections are great when you have a high contrasted scene with fast moving subject which are far away, for example, birds in flight.
Auto-focus modes and “back button” focus
There are two types of focussing available, single shot and continuous. Your camera starts the auto-focus process when the shutter button is pushed half way down or when the AF button is pushed. When using the AF button, the auto-focus process is not hindered by any shutter activity as it utilizes a separate AF button. Using this very handy function will definitely improve your auto-focus, especially with moving subjects. However, you need to use the correct auto-focus type for different subject types.
One Shot or Single AF One shot (Canon) or AF-S (Nikon, Pentax etc.)
This AF mode simply focusses and locks the focus whenever you auto-focus. It is great for portraiture, landscapes or scenes where there are static elements. Most cameras will emit a “beep” sound when the focus has been reached. If you keep your shutter half way down (for AF) or the AF button depressed, the focal distance will be locked by the camera until the shutter is released. Once the photo has been taken, the auto-focus process has to start again. It is handy for tricky compositional situations where you want to recompose a scene but lock your focal distance. Simply focus, recompose and then take the photo.
Continuous Auto-focus or AI Servo AI Servo (Canon) or AF-C (Nikon, Sony, etc.)
Continuous auto-focus modes allow the auto-focus system to focus on the scene non-stop. This is a great mode to use when there is movement in the scene. AI Servo or AF-C tracks focus on moving subjects so that the auto-focus always stays accurate regardless of whether the subject has moved or not. Using your back button AF in conjunction with this is a very useful technique in getting sharp, moving subjects. In the burst below, the hippo charged us and covered a good 50 meters in a flash. I captured the series of images using continuous AF and managed to get every photos focus sharp even though the focal distance was continually changing. I have only shown 6 images here for illustrative purposes. This is the most important AF mode in wildlife photography due to the unpredictability of the subjects.