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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?

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A non-trivial part of it is "mine is bigger" and thus its better if you have 61 instead of 51. Silly marketing, like mega-pixel wars of old. – Pat Farrell Apr 20 '12 at 4:49
up vote 23 down vote accepted

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.

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Yes, but how of a difference does it make in practice between having say 399 focus points (in the A7R II) vs 100 (in my NEX-5R)? At what point would you say further improvement isn't noticeable in normal use? – Vaddadi Kartick Oct 18 '15 at 3:50
I guess you have to define "normal use." Without understanding the differences between those cameras or their capabilities, I bet shooting action is a lot more successful with more AF points simply because of the precision offered. If that's "normal use" I would say the improvements are likely noticeable. – Dan Wolfgang Oct 18 '15 at 10:37
By normal use, I meant common use cases for non-professional users. Imagine the kind of photos people take normally: landscapes, portraits, street scenes, nature, cityscapes, night photography, and so on. Does 399 vs 100 focus points make a difference for these? Or is it only shooting action where the improvements are likely noticeable? For example, my camera often mis-focuses at night. Not focuses on the wrong thing, but the entire scene is blurry and completely mis-focused. More focus points seem unlikely to help this situation. – Vaddadi Kartick Oct 18 '15 at 11:50

With my Pentax K10D, which has only 11 AF points, I find that if I'm tracking a small object, like a distant bird, it's possible for the bird to fall into a gap between AF points, or off the edge of the pattern. This results in the AF system "hunting" all over the place for something to focus on, and I can't even see the bird to follow it anymore.

A large number of control points allows them to be packed densely, so that a moving target moves smoothly from one point to the next, without falling into a gap.

(You could also solve the "gap" problem by making each AF point sensitive to a larger area, but that would make it harder to tell what the system is focusing on. You might think it's focusing on a portrait subject's eyes, but it's actually focusing on the nose.)

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You may not need to use all of them at once - The best part is that you will be able to use them in different pattern. Like only right, only left, only top etc, and many more. And doing that, you will get it faster and the focus accuracy will increase.

These things are helpful to shoot photo in critical conditions, like when you are shooting a horse in a race track, or a biker moving fast. This is just an example, and there are many more where you will find these increased number of focus points and the patterns are tremendously handy.

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