Canon 5D Mark III Experience
4.3b Spot AF (Manual Selection)If you need to focus on a precise area that is smaller than the Single-Point AF area, you can make use of Spot AF. This will not necessarily make your focusing more accurate in general situations just because it is smaller, but rather it is used to address specific autofocusing challenges. For example if you wish to focus between the parts of a fence or cage to a subject beyond, you may find that Single-Point AF searches back and forth between the near fence and the further subject, because the area it is looking at to find the subject encompasses both potential subjects. Spot AF will allow you to target in on a more precise area. Although Spot AF is indicated in the Viewfinder by the tiny square within the larger selected AF Point square (see Figure 69), Spot AF will actually pinpoint the focus to an area about the size of the larger square.
Figure 69 - Simulated view of viewfinder, using Spot AF Autofocus Area Selection Mode.
Spot AF is ideal for situations such as focusing on a bird in a tree. Single-Point AF will likely look at an area that may include surrounding or overlapping leaves, or closer branches and thus might focus there, but with Spot AF you have a better chance of focusing precisely on the intended bird. While Spot AF will be more accurate in certain situations as described, it should not be used for general use. Because it is so precise, the area it looks at to find contrast or a detail on which to focus may be an area of solid color. For example if you used Spot AF to quickly focus on the general cheek and eye area of a face, it may be aimed at an area of skin without contrast, whereas the Single- Point AF area might encompass the cheek and the eye and thus find enough contrast to be able to properly and quickly focus. Again, the camera will track the focus distance of a moving subject that remains under the selected AF Point, but will not pass focus tracking over to the adjacent AF Points if the subject moves away from the initial AF Point or you are unable to keep your subject located at that point. For that you will need to use one of the AF Area Selection Modes other than Single-Point AF or Spot AF.
4.3c AF Point Expansion (Manual Selection) - 4 AdjacentIf it is too difficult to place a single AF point on a subject because you need to work quickly or the subject is moving too quickly or erratically and may stray from the selected AF Point, you can focus on your subject using AF Point Expansion, which will also make use of some or all of the immediate surrounding AF Points to find or retain focus on your subject. AF Point Expansion - 4 Adjacent will make use of the 4 points
