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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.

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This isn't an answer per-se, but I just wanted to comment and say that I have been working professionally as a photographer for ~15 years now and for better than 95% of my shots I only use the center point for AF. In fact, the only time I really use anything else is if I'm working on a tripod and I have the composition locked down and the focal point happens to be off-center... Anyway, like I said- not really a standalone answer, but I did want to validate your question and your instinct... –  Jay Lance Photography May 18 '11 at 4:39
    
There are also reasons to avoid selecting another focus point on low-end cameras. On my Canon 500D, other AF points seem to be of much lower quality in comparison with the center AF point. For a while I tried to use other AF points, but I was disappointed with the results. Even if I shoot something that is "easy" (clear contrasts, fairly flat object, nothing distracting anywhere near the AF point), and even if I get the AF confirmation, far too often I got photos where the focus was off. I switched back to the center point and focus & recompose, and it works exactly as it should work. –  Jukka Suomela May 18 '11 at 10:40
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@Jukka usually the center point is, especially with the widest apertures, a cross-type AF sensor (see: Understanding Camera Autofocus @ Cambridge in Colour). I used to compose the scene, then select the focus points I wanted to be in focus, then focus and take the picture. Nowadays I usually focus the elements that I want to be in focus first with the center point and then recompose & take the picture -- at least with AF-lenses. –  koiyu May 18 '11 at 12:04
    
True, but on the 7D mentioned, they are all 19 points are cross-type –  Robin Jan 9 '13 at 20:58

9 Answers 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.

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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.

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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).

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+1 "AI Servo" + "Focus and Recompose" = PITA! –  ysap May 18 '11 at 17:12
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@ysap unless you change the AF-trigger button away from the shutter –  koiyu May 18 '11 at 18:23
    
@koiyu +1 comment! LOVE my back-button focus! Couldn't work without it... –  Jay Lance Photography May 19 '11 at 20:09
    
@koiyu , I wasn't aware of that. I think I'll give that a try! –  ChrisFletcher May 20 '11 at 13:34

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.
enter image description here
One second later:
enter image description here

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I believe that the OP asks with respect to manual selection of the AF point. Tracking BIF is done with the one of the automatic modes. –  ysap May 18 '11 at 17:11
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Did it survive the crash? :P –  Andrei Rinea Oct 15 '11 at 9:08
    
@Andrei Rinea - What, the Pelican (the bird)? That's how they fish for food (diving into the water). It was fine. –  Fake Name Oct 15 '11 at 9:17
    
I was joking, hence the ":P " –  Andrei Rinea Oct 15 '11 at 9:36

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.

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exactly what I was thinking –  MattiaG May 18 '11 at 8:46
    
I experienced this yesterday night, taking photos of the moon. –  MattiaG May 18 '11 at 8:47
    
@Mattia Gobbi: If you're taking pictures of the moon, you could also just manually focus to infinity and forget the AF system. –  Henry Jackson May 18 '11 at 12:52
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@Henry - no way, that gets me out of focus. At least on my tamron 70-210mm 1:3.8-4, which only has manual focus, infinity is infinity :-) It is hard to see if things really are on focus at night, so you need the camera to tell you focus is ok, and as the clouds are moving you want your shots to be already framed around the moon, to shoot at the right moment w/o adjusting the tripod head. –  MattiaG May 18 '11 at 13:28
    
@Mattia: I haven't tried this, but I'm surprised that the moon doesn't count as "infinity" for focusing. How much further away would you want to focus?? When I get a clear night I guess I'll try this with my lenses, but this is not what I would expect. Perhaps your lens is out of alignment? –  Henry Jackson May 18 '11 at 16:43

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.

enter image description here

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.

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Naturally, this discrepancy be more noticeable with shallower depths of field (large aperture, close focusing, etc). In addition, many lenses have a field curvature, so you can't exactly predict how the focus plane will fall across the field. There is also focus shift, but that is something that this can't fix. –  Eruditass May 18 '11 at 6:16
    
+1 this point was explained very well in Zack Arias' recent weekend workshop @ CreativeLIVE. –  ysap May 18 '11 at 17:16
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+1 I don't fully understand this, but I like the picture. –  Xeoncross May 18 '11 at 19:20
    
@Xeoncross - See it better from another picture? Here: i.stack.imgur.com/SjI4A.jpg and if JerryCoffin would like to use that picture then be my guest, I made it for this question but your answer is already saying the same that I would have posted. –  Esa Paulasto Feb 15 at 8:06
    
Amount the focus plane is off equals: ∆f = x*tan(alpha/2)*tan(alpha), where x is the initial distance from the subject, and alpha the angle of rotation. –  Martijn Courteaux Jun 5 at 18:53

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.

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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.

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+1 because even the simplest "action" suffices: taking a picture of a smiling newborn where the face is off-centre (e.g. capturing mothers and childs faces in one picture) and you wouldn't have the time/nerve to recompose. –  Leonidas May 18 '11 at 20:58

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.

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