Why so many points in AiAF?


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Jun 18, 2005
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#1
Cameras these days have many-point-AF, for example, Canon has it's 9-point AiAF.

However, what is the purpose of having so many points? In the end, isn't the focus on one plane only?

Please enlighten thanks.
 

reno77

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#2
Focusing is not on 1 plane. Imagine you are taking a closeup of a person's face.
If you only had center focus and use a shallow depth of field, only the nose will be sharp.
If you have focus points on the corner, you can focus on the eye and make the nose blur.
 

SnapJaX

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#3
it is mainly more catered for those who know nuts about how to focus correctly..
hence the term "point and shoot" comes to mind...
 

Jun 18, 2005
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#4
I'm thinking it's a gimmick? Is it not? Even the human eye focusses on one thing at a time, are these cameras just trying to show what will be in focus?
 

cantaresg

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#5
Basically those are the points where the focusing sensors are located. By having more autofocus points, it enables a higher chance of getting the subject focused, especially in the AI servo mode.
 

dorts

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#6
I actually don't really bother with the amount of AF points. I usually only focus using the center point. I just point the center at the place I want to focus, then recompose the image. :)
 

zac08

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#7
Cameras these days have many-point-AF, for example, Canon has it's 9-point AiAF.

However, what is the purpose of having so many points? In the end, isn't the focus on one plane only?

Please enlighten thanks.
Well, it balances the aperture choice (which gives the DOF) to the centre of the whole scene ensuring that those faces would be within the DOF zone.
 

gryphon

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#8
It is super useful especially if you are taking fast action stuff.
Using AI Servo in sports, with many AF points, the camera can autofocus and track the subject better. Thats 1 reason why Canon 1 series rules in sports photography. If you watch EPL, you will notice 80-90% of long lenses photographers use are white!
 

grantyale

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#9
It is super useful especially if you are taking fast action stuff.
Using AI Servo in sports, with many AF points, the camera can autofocus and track the subject better. Thats 1 reason why Canon 1 series rules in sports photography. If you watch EPL, you will notice 80-90% of long lenses photographers use are white!
Exactly. For quick snaps and tracking - the AF simply picks the nearest subject with sufficient local contrast. I find the 9/11 point on the current Canon/Nikon mid-range adequate for quick snaps without having to manually select one or worry about losing the shot with focus-and-recompose. (However, the joystick on 20D/30D is still a BIG plus for direct selection of AF point.)

That said, the curious Pentax K100D (dirt cheap) has 11 AF points and 9(!) of them are of the cross-type. BUT - it selects the center point 90% the time regardless of composition.
 

melvinmah

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#10
I actually don't really bother with the amount of AF points. I usually only focus using the center point. I just point the center at the place I want to focus, then recompose the image. :)
Yea that's the way it should be but multiple points does help newbies. Come to think of it, many DSLR users I see these days don't know how to use the focus and recompose technique (which is a basic). Even DSLR have face detection now hehe.
 

melvinmah

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#11
Exactly. For quick snaps and tracking - the AF simply picks the nearest subject with sufficient local contrast. I find the 9/11 point on the current Canon/Nikon mid-range adequate for quick snaps without having to manually select one or worry about losing the shot with focus-and-recompose. (However, the joystick on 20D/30D is still a BIG plus for direct selection of AF point.)
It won't be 100% accurate if you've multiple moving objects. Best is to be on continuous AF/servo, focus on the subject and keep it in frame.
 

Stratix

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Oct 13, 2005
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#13
the multiple points for AF aides when you compose your image off center. Nothing with focus lock and recomposing, but i seldom do that, unless the subject is hard to focus (due to insufficient light, not enough texture for it to lock focus etc).

where possible, i'll use the joystick to select the focal point put my subject closest to the AF point. This way, i find that i could lock focus more consistantly and more accurately, especially with fast big apeture lenses.

but then again, thats me. perhaps i dont know how to use my camera after all. =)
 

Andy Ang

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Jan 10, 2006
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#15
To me, it has no use, but one. When you ask passer-bys to take a picture of you. Make sure you turn it on. ;-p
 

grantyale

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#17
Yea that's the way it should be but multiple points does help newbies. Come to think of it, many DSLR users I see these days don't know how to use the focus and recompose technique (which is a basic). Even DSLR have face detection now hehe.
It doesn't take a newbie to appreciate the advantage of multiple AF points. :bsmilie:
 

grantyale

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#19
maybe you can share some of the advantages with us?
Come on - that's just a joke :sweat: As has been pointed out, it helps when focus-and-recompse doesn't work well and when one's tracking a moving object.
 

Jun 18, 2005
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#20
Then what about the application to point and shoot cameras? It automatically chooses a few points that are on the focus plane, hence showing what will be in focus, and that's all?
 

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