How do dozens of "focus points" help you get better pictures

[allegretto]

I'm just a fun-loving amateur photographer. I still focus the on my main object, lock it and then proceed to compose and shoot (when I have the time). When hurried, I still try to ensure focus first.

But the modern DSLR's have dozens of focus points located all over the screen. It seems to me that this could result in focus "confusion" when on the run and perhaps compromise important shots. So, I tend to just set it to focus on a central point and do what I've been doing. When I've tried the multi-points it leads to all manner of boxes lighting up, and then larger boxes... etc.

So, for those of you who take photos for a living; do you use these muti-points? and if you do, when/where/how do you use them to help you?

Sorry for the complex question... but the whole question confuses me. 

[RazorTM]

If you're tracking birds in flight and for example have a flight of 4 birds which are staggered diagonally across your viewfinder, with the bottom-right bird being the closest, you can easily choose a focus point to keep that bird in focus and preserve your framing.  Otherwise, you would have to center the bird in the viewfinder and possibly cut the top left bird(s) off.  The other alternative is to zoom out (not always possible since birders usually have a big prime lens) and crop in post.

When shooting portraits at f/1.2, focus and recompose usually doesn't work very well, so having a focus point right on the eye of the subject is a big help.

[torger]

The camera's auto focus system use many focus points to be able to track a moving subject. You may initiate your tracking focus with only one point, and then the camera use all the others to track it when the subject moves across the frame. This is useful for sports photographers.

[stamper]

If it is a Nikon camera then Nikon states that the focus points on the periphery aren't as accurate as the the central ones. 

[BernardLanguillier]

On some cameras with advanced AF, the ability to have the camera focus automatically anywhere in the frame on the closest subject in the image is a godsend when you need to take a picture very quickly to capture an event lasting only a fraction of a second... like a child smiling. 

The choice is basically trust the camera AF or don't get the shot anyway.

Cheers,
Bernard

[Nick Walker]

Multiple focus points can be useful for erratically moving subjects where the photographer cannot guarantee holding focus, e.g. on a persons head or chest. Football is one of the hardest sports for AF systems.

With experience of Canon (up to 1D MKIII) and Nikon systems I have found that multiple points are useful, in the case of the Nikon D3 series, 9 AF points (very occasionally 21). It depends on the sporting discipline, more often than not I only use one AF point but always select the single AF point to suit the composition in relation to the pre-visualised image in print. For a double page spread, subject to the left of centre fold, I would pre-select an AF point on or nearer the left. With 45 (Canon) or 51 (Nikon) activated AF points the camera slows down and makes focusing choices (based on contrast) on my behalf - something that I do not want.

Central AF sensors are useful under certain circumstance to take advantage of the superior cross type sensors - poorly lit/low contrast conditions and converter use (outer AF is not as accurate with converters). In good light with good contrast there are good reasons not to use the central AF sensors. It encourages sloppy composition and wasted pixels with landscape images where the subject is small in the frame and meant to be near the rules of thirds - this was so evident with early AF cameras that only offered a central AF sensor and was much talked about with colleagues. I have never had any issues with Nikon's D3 series off centre AF points, even though they are not cross type sensors, with super-telephoto fast aperture lenses used wide open.

Where depth of field is small the best practice is to frame the subject first, then select an AF point, not select the central AF sensor then recompose. Less of a problem at greater camera to subject distance (still not best practice) but more room for focusing error on a tightly shot portrait where the subject's head is say at a slight angle to the camera. In these circumstances it is better to compose the portrait and then select an AF point which happens to fall over the eye or is the closest to the eye - no recomposing or barely any.

AF systems should work out of the box for general subjects. However a good understanding of a cameras AF menu options (plethora of choices) is important to get the best out of erratically moving subjects, (speed and direction changes) and where the subject maybe temporarily obstructed - team games, posts and poles (downhill skier), or routinely disappears (breast stroke swimmer). These challenges can be met by varying the tracking sensitivity (Canon), focus tracking with lock-on (Nikon).

On a side note, using the back button can be more versatile than relying on the shutter button for AF.

Less chance of inadvertently tripping the shutter than using the shutter button during AF checks - think golf.

Allows the photographer to stay in servo/tracking mode but instantly lock focus by merely releasing the AF button to disengage the AF.

Rather than fire the shutter button remorselessly during tracking, the back button only activates AF whilst 'you' choose the shutter timing - being selective helps camera buffer.

 http://www.nikonusa.com/pdf/manuals/dslr/D3ProTechnicalGuide.pdf

PS: Before autofocus technology it took junior photographers' approximately 2 years on average to become fully proficient at manually focusing football. A lot of film still ended up in the bin due to focusing errors.