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What is an AF Point?

The Basics:

An AF point is a specific part of the frame where the camera is able to focus on something. When looking through the viewfinder, these points are denoted by small squares clustered around the centre of the frame. Generally speaking, more expensive cameras have a larger number of AF points, which affords the photographer more effective auto-focus capability.

A Little More Detail:

EOS DSLR cameras can have anywhere between 9 and over 60 AF points, depending on the level of the product. The photographer can choose to use a single specific AF point (Single Point AF), or choose to let the camera decide for itself which points to use, based on the closest subject (Automatic point selection). Cameras higher up the range also allow you to cluster points together, switching between different points around the frame, depending on your creative intent (Zone AF, and AF Point Expansion).
Also, it’s important to note that not all AF points are the same. 

For example, a ‘Cross Type AF Point’ is more sensitive, and therefore more effective than a standard AF point. Similarly, a ‘Dual Cross AF Point’ is even better.

Additionally, you’ll also hear AF points being referred to as being sensitive to a particular aperture. The larger the aperture sensitivity, the more accurate the focus will be. However, you can only focus with a f/2.8 sensitive AF point if the lens supports that aperture. Many lenses, particularly entry and mid range, have a maximum aperture of f/4 or f/3.5. This is why most AF points on an AF sensor are typically f/5.6 sensitive.