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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

 
CaptivatedByBeauty
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Apr 19, 2011 07:08 |  #46

deronsizemore wrote in post #12246454 (external link)
Sorry for jumping on on this thread, but where should the AF point be if not on an eye? Seems like everything I've read recommends having it on an eye?

Maybe my point above was not clearly put. I was not suggesting not focusing on the eye. Or put another way, I was suggesting focusing on the eye.

But, the question is, how to focus on the eye. If you use an outer AF sensor, and put the little black square on the centre of the eye, you may not be focusing on the eye, because the actual AF sensor is not where the black square is in the viewfinder.

In my case with a 60D, to focus on the eye, I need to put the area just inside the black square on the eye. That makes it a bit of a guess.

If you look at this thread you'll see what I mean:

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

Here's an example:

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You'd think the top AF sensor would have focused on the pot behind. But the actual sensor is lower than the square, so it's focused on the lego in front.

Steve
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Have: Canon 5D mkII, Canon 60D gripped (DBK), Canon 70-200mm f/2.8 L IS USM mkII, Canon TS-E 24mm f/3.5 L II, Canon 1.4x mk II Extender, 1.25/2.5x Angle Finder, Triopo GT-3229X8.C Tripod with B2 head

  
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tkerr
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Apr 19, 2011 09:32 |  #47

If I am not mistaken, evaluative metering is linked to the selected AF point while others aren't.
If you're using anything other than evaluative metering the camera is going to meter the exposure at the center and not your selected point of focus.
So, How much, if any, effect will that have on the image sharpness at your AF Point?

Just a thought!


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deronsizemore
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Apr 19, 2011 12:42 |  #48

CaptivatedByBeauty wrote in post #12251547 (external link)
Maybe my point above was not clearly put. I was not suggesting not focusing on the eye. Or put another way, I was suggesting focusing on the eye.

But, the question is, how to focus on the eye. If you use an outer AF sensor, and put the little black square on the centre of the eye, you may not be focusing on the eye, because the actual AF sensor is not where the black square is in the viewfinder.

In my case with a 60D, to focus on the eye, I need to put the area just inside the black square on the eye. That makes it a bit of a guess.

If you look at this thread you'll see what I mean:

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

Here's an example:

QUOTED IMAGE

You'd think the top AF sensor would have focused on the pot behind. But the actual sensor is lower than the square, so it's focused on the lego in front.

Interesting. Thank you for the clarification!

Typically I just use the center point. Does what you're saying only apply to the outer points? Or if I have my center point directly over the subject's eye, could the focus point actually be lower/higher?


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CaptivatedByBeauty
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Apr 19, 2011 17:12 |  #49

deronsizemore wrote in post #12253470 (external link)
Interesting. Thank you for the clarification!

Typically I just use the center point. Does what you're saying only apply to the outer points? Or if I have my center point directly over the subject's eye, could the focus point actually be lower/higher?

All the things I've read, ie what other people have found when testing, suggests that the centre AF point is in the centre and fairly close in size to the black square. I think that covers the 60D, 5d mk II and 7D, and by implication the other Canon cameras with the same sensors.

And from comments about people preferring the accuracy of the centre AF point also suggests that the centre AF sensor is where you'd expect it to be!

So yes I think it only applies to the outer AF sensors.


Steve
CaptivatedByBeauty (external link)
Have: Canon 5D mkII, Canon 60D gripped (DBK), Canon 70-200mm f/2.8 L IS USM mkII, Canon TS-E 24mm f/3.5 L II, Canon 1.4x mk II Extender, 1.25/2.5x Angle Finder, Triopo GT-3229X8.C Tripod with B2 head

  
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CaptivatedByBeauty
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Apr 19, 2011 17:23 |  #50

tkerr wrote in post #12252229 (external link)
If I am not mistaken, evaluative metering is linked to the selected AF point while others aren't.
If you're using anything other than evaluative metering the camera is going to meter the exposure at the center and not your selected point of focus.
So, How much, if any, effect will that have on the image sharpness at your AF Point?

Just a thought!

An interesting point. I did some more tests on the 60D AF sensors recently, some of which involved using the centre AF point and recomposing to compare the result with using an outer AF point. A light face and dark background. Of course when recomposing the metering changed because more dark background appeared towards the centre.
That's why I was in manual :) and why I tend to use manual for the less dynamic shots.

So my thinking is:
1. If shooting dynamic situations, especially with varying light levels, and moving subjects, select the right AF point and use Av mode with evaluative metering.
2. If shooting a more static non-changing light situation, use the centre AF point and recompose, or use Live View at 10x and manually focus, and set the exposure manually.


Steve
CaptivatedByBeauty (external link)
Have: Canon 5D mkII, Canon 60D gripped (DBK), Canon 70-200mm f/2.8 L IS USM mkII, Canon TS-E 24mm f/3.5 L II, Canon 1.4x mk II Extender, 1.25/2.5x Angle Finder, Triopo GT-3229X8.C Tripod with B2 head

  
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center AF vs 9-point AF
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