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FORUMS Photography Talk by Genre General Photography Talk 
Thread started 07 Apr 2011 (Thursday) 22:37
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center AF vs 9-point AF

 
tkerr
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Apr 08, 2011 14:49 |  #16

krb wrote in post #12184145 (external link)
Is the title of this thread "center AF vs 9-point AF" or is it "center AF point vs off-center AF point"?

The title and the question aren't necessarily the same, or could at the least be misinterpreted. Especially since you cannot actually use all 9 AF Points at once. As already pointed out, activating all 9 point vs selecting a single AF point allows the camera to make the choice for you, which can and often will choose the one you don't want.

ekinnyc wrote in post #12180682 (external link)
the T2i has 9-point AF, however, i keep mine on center point AF always. this way, i can pick what i want to focus on, and recompose as necessary.

am i giving anything up by using center point AF 100% of the time?

Yes and No; It depends on the situation! When shooting stationary subjects/objects, using the center AF Point to acquire focus then recomposing the shot works fine and is probably the most accurate focus point for most lenses. But then you would probably be using a tripod and could be focusing manually.
On the other hand, if you are not shooting stationary subjects/objects where you are not able to focus then recompose the shot, E.g. See above photo, then one of the other AF points might be advantageous for you.

There will be a number of situations when using one of the other AF Points is a better choice. E.g. Moving subjects and/or a very narrow DOF.

Other advantages to using one of the other AF points, more specifically the six surrounding the center point, is that you can frame/compose your shot on one of those AF points utilizing the rule of thirds; rather than centering the subject on the center AF point then cropping the picture in PP. You might not want to crop that much to create a properly composed picture and using one of the other AF points can save you the trouble.

Most of my photography is outdoor stuff such as Birds which are constantly on the move. I have tried turning on all 9 AF Points. It's a big PIA, I wouldn't recommend it.


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Yasa
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Apr 08, 2011 18:09 |  #17

Is there a chart/scale/equation for determining the inefficiency of the outside AF points compared to the centre? I mean, why is the centre more reliable than the outer-region? On top of that, is there a continual degradation of focus-quality from the centre outward in a 45-point focus?


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krb
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Apr 08, 2011 18:14 |  #18

Yasa wrote in post #12185486 (external link)
Is there a chart/scale/equation for determining the inefficiency of the outside AF points compared to the centre? I mean, why is the centre more reliable than the outer-region? On top of that, is there a continual degradation of focus-quality from the centre outward in a 45-point focus?

The center is more reliable because there is less simply less distortion.

And yes, the error increases as you move to the edges.


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shoturtle
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Apr 08, 2011 18:19 |  #19

the center point is optimized for f2.8 or faster lenses. An on the rebels it is the only cross type point. So it is the most accurate one for AF.


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Yasa
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Apr 08, 2011 18:19 |  #20

krb wrote in post #12185508 (external link)
The center is more reliable because there is less simply less distortion.

And yes, the error increases as you move to the edges.

Sorry for sounding ignorant on the subject, because...well, I am, but what causes the distortion? Is it lens-based or camera body-based? Is there a ratio of sorts that you can compare the amount of distortion between centre and furthest outer-point? Aaaand, is there a correlation between focal distance, distortion and chosen AF point?

Again, sorry for the questions. AF is an interesting subject.


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Sp1207
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Apr 08, 2011 18:42 |  #21

krb wrote in post #12185508 (external link)
The center is more reliable because there is less simply less distortion.

And yes, the error increases as you move to the edges.

I'm not sure if this is all that accurate. Most modern lenses have very little distortion, and the AF sensor will still AF accurately in spite of any fisheye (or lesser) effects, so I don't think it's a factor. Some lenses have field curvature, but again that doesn't affect AF accuracy.

The biggest issue with the outer sensors is that the rebels only have one cross-type AF sensor -- in the middle. The outer sensors are just as accurate (within 1/2 the DoF of the lens), but slower (only compared to using the middle point with a 2.8 or faster lens) as they're not cross-type.


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tonylong
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Apr 09, 2011 02:55 |  #22

Yasa wrote in post #12185486 (external link)
Is there a chart/scale/equation for determining the inefficiency of the outside AF points compared to the centre? I mean, why is the centre more reliable than the outer-region? On top of that, is there a continual degradation of focus-quality from the centre outward in a 45-point focus?

It's simply the fact that the center point has a cross-type sensor which can more easily pick up points of contrast and detail. In decent light you can pick up a decent point of detail and get focus. It can help if you can distinguish between the sensor types you are using, although for practical purposes just aiming for an area that has sufficient detail is sufficient. Be aware that a straight line can thwart you if it's going in the wrong direction.

shoturtle wrote in post #12185528 (external link)
the center point is optimized for f2.8 or faster lenses. An on the rebels it is the only cross type point. So it is the most accurate one for AF.

Is the Rebel equipped with the "f/2.8 advantage"? I'm not sure. But with cameras that are, that can help, especially when you are dealing with low light and action stuff. But for general shooting I wouldn't worry about it much. It is, though, a good reason to settle on f/2.8 or faster lenses for when you need that extra accuracy.

Sp1207 wrote in post #12185642 (external link)
I'm not sure if this is all that accurate. Most modern lenses have very little distortion, and the AF sensor will still AF accurately in spite of any fisheye (or lesser) effects, so I don't think it's a factor. Some lenses have field curvature, but again that doesn't affect AF accuracy.

Yeah, the distortion thing could only apply if your lens has noticeable distortion. I'd rather not have a lens like that!

The biggest issue with the outer sensors is that the rebels only have one cross-type AF sensor -- in the middle. The outer sensors are just as accurate (within 1/2 the DoF of the lens), but slower (only compared to using the middle point with a 2.8 or faster lens) as they're not cross-type.


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CaptivatedByBeauty
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Apr 09, 2011 03:49 |  #23

It's not just a matter of speed/accuracy, it's also knowing where the outer AF sensors really are compared with the claimed positions/sizes in the viewfinder:

AF Sensor Behaviour - More complex than it might appear
https://photography-on-the.net …/showthread.php​?t=1009080


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Apr 09, 2011 04:02 |  #24

Taking Tim Kerr's picture as an example, with the AF point right on the eye, which many people assume is the correct thing to do, I wonder where the focus actually was. On my camera it would probably have been the cheek!
Tim, what camera did you use, and how about a crop of just the eye/cheek area so we can see where the focus was? :)


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tkerr
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Apr 09, 2011 10:56 |  #25

CaptivatedByBeauty wrote in post #12188192 (external link)
Taking Tim Kerr's picture as an example, with the AF point right on the eye, which many people assume is the correct thing to do, I wonder where the focus actually was. On my camera it would probably have been the cheek!
Tim, what camera did you use, and how about a crop of just the eye/cheek area so we can see where the focus was? :)

I know focusing on the eye isn't the right place to aim the AF point, but she was a moving target coming toward me. I was also stepping back trying to maintain a distance at the same time adjusting exposure, and the focal length trying to keep her framed in the shot..
Trying to keep the AF point on the cheek wasn't exactly easy. Just keeping it anywhere on the face was a task.

That was shot with a 50D and EF 28-135mm Kit lens at 100mm.
f/5/6, ISO 200, 1/125s, w/flash, hand held

800x580 100% crop


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CaptivatedByBeauty
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Apr 09, 2011 17:14 |  #26

Hi Tim,
Please don't take what I wrote in any way as criticism. My point was purely that it looks like you'd focused on the right point, but with the outer AF points it's a bit of a lottery as to where focus really is.
I still don't know what the best method is for my camera (60D). Use an outer AF point and try and guess where it's going to focus, or use the centre point, which is a lot better defined in position and size, and is more reliable, and try and find something to focus on which is the right distance away.
Wouldn't be it great if you could simply move the centre AF point around, and select the spot you want to use! And the little square would move exactly to show where the sensor was :) I don't know whether any camera manufacturer has implemented such a system.

PS Your focus looks pretty good :) Difficult to tell whether it's a little front focused on a face as there's not much to see it with. But it must be close.


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tkerr
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Apr 09, 2011 17:33 |  #27

CaptivatedByBeauty wrote in post #12190696 (external link)
Hi Tim,
Please don't take what I wrote in any way as criticism.

I didn't!

CaptivatedByBeauty wrote in post #12190696 (external link)
Wouldn't be it great if you could simply move the centre AF point around, and select the spot you want to use! And the little square would move exactly to show where the sensor was :) I don't know whether any camera manufacturer has implemented such a system.

Now that would be nice!

CaptivatedByBeauty wrote in post #12190696 (external link)
PS Your focus looks pretty good :) Difficult to tell whether it's a little front focused on a face as there's not much to see it with. But it must be close.

I had went through and set the AF microadjustment for each lens, but I had it turned off this time so it probably does look a little to the front. But running around out in the yard trying to take pictures like this, I don't think it would have mattered.


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Apr 09, 2011 18:09 |  #28

One thing to bear in mind -- there is a difference between on-the-fly handheld shooting, like Tim mentions, and in which "acceptable sharpness" has a broader interpretation. and "critical demand" shooting in which you want to take strides to optimzize that critical sharpness in your technique. Not that we should ever be sloppy, and I still tend not to center-point focus and recompose, but I can't afford to be too picky when I'm shooting something on-the-run.


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ekinnyc
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Apr 11, 2011 08:38 |  #29

i was shooting a friend's nephews 1st birthday so naturally there were many kids running around. i mostly used center AF, but i did try to use outer points.... maybe i should have tried using it on kids running around, but i was shooting f/4 or above, so i think i was able to get away with focus center and recompose


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pbelarge
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Apr 11, 2011 08:59 |  #30

tonylong wrote in post #12184124 (external link)
Like the portrait shown above, you can get the right framing for a subject and not have to bother with recomposing. I do it all the time. Of course, shooting with a narrow depth of field can make this mandatory, and I often shoot with a pretty narrow depth of field, but in general I like to keep my subjects properly framed and that's what the outer points are for.



I am not a big fan of recomposing...maybe because I am not good at it. I use the same method as Tony does above.


just a few of my thoughts...
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