autofocus - What is the advantage of the huge number of AF points?

If the camera has only 1 central AF point, it is difficult to focus on an off-center subject (without recomposing after autofocus) so having 3 AF points seems really useful. Similarly I understand the advantage of having maybe 5 or event 9 AF points.

Nowdays flagship DSLRs have huge number of AF points. For instance Nikon D800 has 51, Canon 5D Mark III has 61. What is the advantage of this? Does using 51 AF points really increases the chance that the camera finds out my intention and focuses on my real subject instead of something else next to it?

Answer

Having many autofocus points opens up the camera's capabilities because it can function in different ways depending upon settings and camera features:

• You can select a single AF point (get precise with exactly which point you want to use, without recomposing at all).


• You can select a group of AF points (a group of AF points means you don't need to be as precise in your selection/aim, and that if the subject moves within the group it'll still maintain focus).


• You can select the best AF point for the job (not all AF points are equal; some are more/less sensitive, and some are horizontal sensitive while others are vertical).


• You can let the camera choose which AF points to use (it'll try to get the most area in-focus at once).


• You can track subjects as they move across the frame (as the subject exits one AF point or group of AF points and enters another, each point will keep the subject in focus).

Throw in intelligent metering systems and the camera can do scene recognition:

• Is this a portrait? (Focus on the face.)



• Is this a group photo? (Try to focus on the most faces.)


• Is this a landscape? (Try to get everything in focus.)


• Is this a sports scene? (Try to use focus tracking on the running person.)

And if you include flash in the mix, those AF points are also used to try to calculate flash exposure based on subject distance.