Why would I want to select an autofocus point?

Question

I'm trying to understand why I would ever want to select an auto focus point other then the center point. I have a Canon XTI Rebel DSLR which has 9 auto-focus points, but I'm also looking into getting a 7D with 19 points.

Most of time I have my camera set to the center auto-focus point. Then when I take a picture I aim the center point at the object I want to focus on, half press the shutter button to trigger the focus, recompose if necessary, and then fully press the shutter button to take the picture.

So why would I ever want to explicitly select another auto focus point? I don't think of wanting a point at the top left of the frame to be in focus, but rather I want a specific object in the frame to be in focus and that object might end up in the top left of the frame. In the next picture I may want that object to be in the top right of the frame, but I don't want to have to fumble around with switching AF points between every shot. It just seems so much easier to simply point center AF point at an object then it is to fiddle with selecting a specific point.

Answer 1

With some lenses (mostly fast wide-angles) there's a possible problem from focusing and then re-composing: as you re-compose, what you originally focused on will no longer be on the plane of focus.

With longer lenses, this is rarely much of a problem, if you're shooting with something like a 30/1.4 on full frame, your subject could be quite a ways out of focus by the time you compose the picture.

This makes the biggest difference when there's a fairly large angle between where you focused and where you shoot. The slower the lens, the more depth of field you have to cover the discrepancy.

Answer 2

Perhaps it would be helpful to select a non-center point if your camera is on a tripod and you have it set up so the framing is just right.

Answer 3

If you're trying to compose your photo so that the subject isn't directly in the center (using rule of thirds or similar composition), the selectable focus point is a big help. You can pick a point to the left or right of center (or any of the other points) and compose around that point.

An alternate technique is focus and recompose, where you focus on-center, and then recompose your photo while holding focus as-is.

Although either of these techniques work equally well in many cases, you might find reasons to use one vs. the other in certain situations. Setting a specific focus point does limit you to that point, which can seem constraining, but when you know you're going to compose in a certain way, it lets you lock focus on that point and shoot very quickly, whereas focus-and-recompose will always require that extra split second on your part to move the camera after focus is locked.

If you're looking at selectable focus points in the XTI vs. the 7D, be sure to check out the improvements found in the 7D's autofocus system. You might find that features like AF Point Expansion make the selectable points a little easier to live with.

Answer 4

The additional AF points are essential for Action/Bird photography.

I have enough trouble keeping fast-moving birds within the viewfinder, let alone keeping the center AF point over the bird. As it is, with the 9 AF points on the 5D2, It's quite challenging to even get a shot in focus.

Take, for example, a Diving Pelican. You have something like 2-5 seconds to notice which pelican is diving (there are generally a few about, at least where I live), get it in the viewfinder, focus, and take the shot.

I seriously considered going for a 1Ds3 over a 5D2, simply because it has better/more AF points (this was before the 7D, and I really wanted FF anyways).

Obn the other hand, when you get a shot in focus, it's worth it.

One second later:

Answer 5

One situation where you want a different focusing point is when you have action shots you wanna take. Imagine a bike race for example and you want the rider in the left part of the picture, then you won't really have time for focus and recompose. Another application is shooting a flying bird.

Of course you could use your AF Servo function to trace a moving object, but sometimes this is not always possible, for example if your background plays a crucial role in your composition too. Maybe think here of a passing car in front of a monument where the monument really should be centered and the car should be in focus when it enters the frame.

Answer 6

You're using a method known as 'focus and recompose' which works well in many situations, however if you're not using the 'one shot' focus mode (using 'ai servo' for example) then there's a drawback to this. As you move the camera to recompose the shot the camera will re-focus to try and keep your original point in focus, which is not what you want.

In this case multiple focus points are very useful, especially as you can change the focus point without removing your eye from the viewfinder, something you cannot do with the focus mode (atleast on my 550D).

Answer 7

I've toyed with this for the purpose of framing a moving baby. I can't center-and-recompose all the time because she's in motion, but I want to keep re-focusing on her face.

Answer 8

For regular shooting the center point method works. But for action photography they somehow find the use a little more. Not really sure how but this is what i heard from a bird photographer. I think it would be nice to use the multi-point system in auto mode rather than a single point selected for best results.

Answer 9

Depending on the camera and the mode, pressing the shutter release half down may also lock in the exposure settings as well as the focus. In that case, you may not want the focus point in the centre when setting the exposure.