Multi-point AF Systems: Purpose?

Hi2all!

Yes, I realize the subject title may appear a bit odd to many, because everyone is aware of the "User Guide" definition of a multi-point AF system, but what I'm looking for is real-world usage.

I have a 50D and if I look at the official specs, quoted here from DP Review:

9-point TTL CMOS sensor
• All points cross-type for lenses of F5.6 or faster
• Center point additionally sensitive with lenses of F2.8 or faster
• AF working range: -0.5 - 18 EV (at 23°C, ISO 100)

Being an old-skool man myself, I never use anything other than the center AF point, choosing the classic "lock on subject and recompose" method vs. fiddling with one of the other 9 (or 45, wth... :! points. Not to mention the fact that most of us here are sporting "fast" lenses with f/2.8 or better max apertures, making it clear that using anything but the center AF sensor is a risky proposition...

Which brings back around to my initial question: what is the [real-world] purpose of multi-sensor AF systems? I would have suggested as a tool for amateur users but seeing as Canon implemented a 45-point AF system in their co-flagship 1D MK IV, I'd most likely be wrong...


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p.s. on another note, I'd have to disagree with Lungdoc's otherwise excellent sticky-thread, "-=What to do if you suspect a focus problem with your DSLR=-" with his recommendation to avoid max apertures during focus tests (2.c.) I need my f/2.8s to focus @2.8 and my 4L to focus @f/4...
In which case, to tie this in with my main subject, all but the central AF point are useless and/or too risky for me most of the time...

Here's a sample of my "max aperture" tests with my 70-200 4L:

IMAGE LINK: http://farm4.static.fl​ickr.com …75981285_c8b8df​efcb_o.jpg

There are a few reasons to use more than one focus point, or at least use another one than the one in the center.

• Your main focal point is not in the center. If you still use the center point, then recompose, you'll shift the focus plane and end up with back focus on the main subject. This effect is mostly noticeable with wide angles and/or short shooting distances, but it's always there.
  
• You can't easily recompose, because the camera is on a tripod, but you want to focus on something off center.
  
• You are trying to track (Servo AF) something that's moving fast and erratically, so using a single point too often implies that the point is not on the target, and the camera focuses elsewhere. Using more than one point (on a 50D it's one or all, but more advanced cameras have more options here), together with automatic handover of focusing from one point to another, may help with this.
  
• You want to track something that's easy enough to follow with one AF point, but the subject isn't in the center of the image.
  
• Some cameras have more than the center point supporting higher precision with lenses of f/2.8 or better (sometimes from f/4). Normal precision should still be within the depth of field, even for a f/2.8 lens.

I suggest that you actually try your outer points before passing judgment on their capability or lack of.

If you are using your lenses wide open, and you haven't run into focusing issues (focus and recompose), you are either shooting with wide lenses and/or the subject is at a fair distance, putting them within the DOF error, using that technique. Then again, maybe you are centering the subject in the frame.

I get faces out of focus when shooting portait orientation, full body, with fast mid/long lenses, and focusing on their midriff (if I used the center point). There are things you can get away with at f4 that you can't at f2, less so on a FF.

apersson850 wrote in post #10166192
...Your main focal point is not in the center. If you still use the center point, then recompose, you'll shift the focus plane and end up with front focus on the main subject. This effect is mostly noticeable with wide angles and/or short shooting distances, but it's always there.

Thanks for the informative reply! Most of the points you listed are clear-cut to me but I'm having trouble understanding the first (and most interesting to me and my general photo subject-matter).

So, let's take something simple, a girl in a field of spring flowers (yawn...) and we want to adhere to the 1/3 rule so she's in the 1/3 section of the horizontal plane of composition (etc.) So I focus on her, (maybe she's 35deg. from our POV) and then turn towards the "center" of my composition. According to your sentence, she will be OOF (Front-focus and assuming a rather shallow DOF ), correct?

I don't understand the geometry behind this, but I will take your word for it...

***EDIT***

bohdank wrote in post #10166411
If you are using your lenses wide open, and you haven't run into focusing issues (focus and recompose), you are either shooting with wide lenses and/or the subject is at a fair distance, putting them within the DOF error, using that technique. Then again, maybe you are centering the subject in the frame....

Ok, looks like you're both talking about the same "problem"... . But to answer your question, I do avoid max aperature unless I need it to mask the background or (less often) I need low-light support. I clearly need to test this myself using a still subject and working with both the center and outer (1/3) AF points @2.8 to see the difference first hand.

Thanks for the clarification

Travelller wrote in post #10166502
So, let's take something simple, a girl in a field of spring flowers (yawn...) and we want to adhere to the 1/3 rule so she's in the 1/3 section of the horizontal plane of composition (etc.) So I focus on her, (maybe she's 35deg. from our POV) and then turn towards the "center" of my composition. According to your sentence, she will be OOF (Front-focus and assuming a rather shallow DOF ), correct?

I don't understand the geometry behind this, but I will take your word for it...

The focus and recompose error is due to Field Curvature of the focus plane:

That's a very neat chart. I didn't realize that there were flat front and curved front lens. Learn something everyday. Guess I should look at my lens more closely

Travelller wrote in post #10166502
...I focus on her, (maybe she's 35deg. from our POV) and then turn towards the "center" of my composition ... looks like you're both talking about the same "problem"...

msowsun wrote in post #10166619
The focus and recompose error is due to Field Curvature of the focus plane:

Thanks for the illustrations I *think* I understood them. If so, I would think that the example I quoted above is exactly taking place in fig. C & F...

As you (& apersson) seem to be referencing Wide angle lenses, am I to assume that bohdank's example is slightly different, referencing the shallow DOFs when working with close subjects and longer FLs (portraits)? Or are both examples based on the same problem?

Thanks to all for the vital info (& for your patience... )

Lenses are corrected to give a focus field as flat as possible, not curved.
Had the focus field been spherical, then you could have recomposed as you liked, without any side effect. But you couldn't take a picture of a wall and get it all in focus.

So assuming the lens can give you a flat focus field, then that flat field no longer passes through the subject, if you focus at the center of the image and then turn the camera to any side. I mentioned wide angle lenses since they allow more turning without loosing the subject out of sight. I made an error in my first post, where I stated you end up with front focus. It's the opposite. Since the flat field of focus kind of swings away, you end up with focus behind the subject. Sorry for the confusion.

With a telephoto lens you can't swing too much, and still keep the subject in the frame. But then the depth of field is shallower, so...
I think bohdank refers to lenses not having a perfectly flat focus field. Such a lens may not get the face in focus, if you aim and focus midriff on a person. No recomposition involved here.

Even if they had a flat focus field the longer lens, shot wide open, at close/mid distances, you still would not get the whole subject in focus. The distance between sensor/lens and the top of the image and the center of the image is not the same. DOF becomes important.

Mind you, we are talking about fast, and usually, mid length (normal) lenses.

Depending on the combination of factors, this can be much more evident in shorter lenses. FF exacerbates the effect since lenses would be used closer to the subject than on a crop, for equal framing (less DOF).

bohdank wrote in post #10167392
Even if they had a flat focus field the longer lens, shot wide open, at close/mid distances, you still would not get the whole subject in focus.

Very true... I have a lot to learn (but this is a good thing; when you stop leanring...)

Thx2all for the enlightenment

p.s. @Bohdan - if I show my GF your gallery she'll freak because Cuba's on the top of her list of places to see (mine's Japan & with some luck, I'll manage it next year). We will however be hitting Miami up in July (which is where I essentially grew up) and for the moment I hope it will be close enough for her

The only time my camera comes off center-point focus is when I am handing the camera to someone else to take a picture of me or me with others. At that time I put all the points on. I just don't want to take the time to explain to them how to capture focus and recompose.
I use center point and recompose exclusively with the RARE exception where I use the green box.

Interesting discussion.

I use all methods. Depends on the situation. Center Focus and recompose is critical for low light with my 5D or 40D.

I just recently acquired a 1Ds Mark II, and was anxious to find out how 45 focus points over a wide field would really be of practical help...

Answer: Servo. The extended focus points were really put to full use and I was stunned by the speed and accuracy.

JH

Travelller wrote in post #10167493
Very true... I have a lot to learn (but this is a good thing; when you stop leanring...)

Thx2all for the enlightenment

p.s. @Bohdan - if I show my GF your gallery she'll freak because Cuba's on the top of her list of places to see (mine's Japan & with some luck, I'll manage it next year). We will however be hitting Miami up in July (which is where I essentially grew up) and for the moment I hope it will be close enough for her

So close, yet so far

jhoag wrote in post #10167593
Interesting discussion.

I use all methods. Depends on the situation. Center Focus and recompose is critical for low light with my 5D or 40D.

I just recently acquired a 1Ds Mark II, and was anxious to find out how 45 focus points over a wide field would really be of practical help...

Answer: Servo. The extended focus points were really put to full use and I was stunned by the speed and accuracy.

JH

My 40D has 9 cross point types with the center one extra sensitive at f/2.8 but in my experience only the center (top middle, middle middle and bottom middle) are really cross type. The 3 on each side are not nearly as good, hopefully not just something wrong with my camera but perhaps I should ask separately to see as the camera is still under a warranty. I tested with vertical-only line targets and only those middle three really appeared to be sensitive to vertical lines using an EF 24-70mm f/2.8L USM lens. Really, the others weren't much better than the horizontal only outside sensors of my Rebel T1i/500D. Same goes for my 1D Mark II which has 7 cross type sensors and the other 38 are horizontal only (and perhaps coincidentally on the 1D Mark II the 7 cross types are in the center: top center, bottom center, center center, two mid top center and two mid bottom center). Using the camera in portrait position instead of landscape orientation matters of course, on both cameras suddenly in portrait mode those lines become horizontal and the focus is fast and accurate on all sensors. Most targets aren't that much of a problem of course, hopefully there is contrast in both directions.

The 45 AF points on the 1D (not 1Ds) are nice because they cover a lot of the 1.3x field. They cover less of the full-frame 1Ds frame. With the 1D I find I don't need to focus and recompose because the 45 AF points let me focus right on my target much of the time.

In AI SERVO mode on the 1D in "ring of fire" mode all 45 points are active, you start in the middle and then as long as you keep the target in the points it will track it. Again on the 1D this works better than the 1Ds as the points cover a larger portion of the frame (same AF sensor on both but a larger image sensor on the 1Ds).