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Old Sep 9, 2003, 1:32 AM   #1
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Default AF on the 10D not perfect

Take a look at this image. It was taken with the center AF point active only. using the canon fileviewer utility you can see that the red focus box is right on top of the girl's legs, and not sticking out over them. However, the AF system managed to focus on the trees way in the background. It looks like it picked up on the background visible through the small gap between the legs.

Taken at f4.0, 1/500.



Full sized un-edited picture at http://www.pbase.com/image/21161747

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Old Sep 9, 2003, 4:57 AM   #2
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You got to be careful with those AF sensors, they think they are smarter than they are! A better spot to have positioned the AF point would have been at the edge of the red shirt and the blue pant... The girl's tummy!

The AF is spot on if used properly (the skinny legs are OK here :lol:




Also the red rectangle is only a general area indication, sometime I noticed the areas on the edge or slighly outside the red outline get focus on and not inside depending on what it thinks should be focused (ie give it some help)
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Old Sep 9, 2003, 10:31 AM   #3
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NHL mentioned what I was going to say. It was said by a Canon employee over at Rob Galbraith's forums that the AF sensor actually uses an area larger than what is denoted as the AF sensor area. Dumb... really dumb. But that is how it is.

I have seen it make mistakes like this before, but not often, and not on something so large (i.e. shore birds.)

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Old Sep 9, 2003, 1:58 PM   #4
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Just wanted to show you guys this picture. I've noticed in the past that the red square is smaller than the actual area it tries to AF in. Good to hear that has been confirmed by Canon.

However, you would think the sensor would try to focus at the distance of the object that covers the largest part of the sensor, not some tiny small part that also happens to fall inside.

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Old Sep 9, 2003, 2:15 PM   #5
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Think about it... The sensor is not that smart! how can it knows that you want the areas on both sides of the AF rectangle to be in focus? It automatically defaults to the center area instead

You got to give the camera some help here... You're more intelligent that it is and you should outsmart it! It only knows 1 and 0 so don't give the camera a multiple choice, ie lock focus in unmistakable area and then recompose :lol:
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Old Sep 9, 2003, 3:23 PM   #6
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Simple in this case. It should determine the number of pixels within the AF rectangle that fall within a distance range. (do a bucket sort). The bucket with the largest number of pixels in it is going to be used to determine the focus distance.

In other words, it should do some kind of majority voting rule.

But apparently it doesn't do this :-) Not a big deal, as long as you know this. Then you can put the focus point somewhere else, or re-compose as you suggested.

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Old Sep 9, 2003, 3:52 PM   #7
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... Not so simple! What if you are shooting @ a skinny stick or a branch in the foreground instead?

It'll be exactly the opposite and you'll like for the AF to pick on the lesser number of pixels in the bucket... (Just like metering once you get used to how it works) and how it can be fooled, all the better! I think it also looks for the more contrasty or more texture area to focus on as well rather than a flat uniform color...

BTW in this case with all 3 horizontal sensors active it would have picked up the correct focusing area instead (majority rule)... :lol: :lol: :lol:
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Old Sep 9, 2003, 11:10 PM   #8
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Also remember, the 10D is a dSLR and unlike all-in-one cameras, it does not use the CCD for focusing... the pixels are still hidden behind the curtain @ this time.

Theses are phase array detectors which not only give the camera the distance info but also far or near as well helping in the predictive AF! :lol: :lol: :lol:
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Old Sep 9, 2003, 11:33 PM   #9
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My vote is that it be consistant. Since I don't expect perfection (that is the 1Ds! Just kidding, but for that price...) I want it to be consistant so I can learn what it does and take advantage of it (or minimize its chance of a mistake.)

I just hate when it seeks in the wrong direction for AF on a flying bird or even a stationary object some times. I really need to do some testing and figure out when and why it does that.

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Old Sep 10, 2003, 1:08 AM   #10
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NHL, yes, you are correct. The camera must do much more sophisticated detection than my simply majority rules suggestion :-) I should remember to press the left most litte button on the back (next to the zoom) when taking a shot like this. I've that button set to use all seven AF sensors.

I'm with Eric though, consistency is good. As long as it is consistent I can predict what the AF system will do, and act accordingly. To me this is part of the fun, learning the camera in and out.

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