Where do you put the focus point???

The actual crosshairs of the AF-sensor (single point) is much larger than that. So it doesn't surprise me that the camera chose the background to lock on and give you a "false" positive on lock.

from the digital-picture.com

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thats whats been bothering me. Her hair/face seems contrasty enough. I know the size of the focus point is bigger than the box, so it could have tried tp grab focus on the little boy's hair also. It could also have tried to grab focus on the passing car behind her, but it seems like it grab focus on the sign above here head which is, what, like 50ft away?

I know this julian, so would you suggest I lower the focus point next time? Focus then recompose? Honestly, I didnt even see the sign above her head until I uploaded my files to my desktop, so I guess in the end I couldn't have avoided it.

Her face has enough contrast... if it was lit. But it wasn't. At best, you only had the street lights illuminating on the outline between her hair and her face. With the Single point focus, I'm pretty sure the algorithm doesn't even attempt to factor in the micro-contrast from the eyes.

What we also don't know (and only you do, and you might not even remember) is whether when you took the shot, did you select that AF point but made sure you took it w/o recomposing? But in that scenario, I wouldn't think it would have mattered much because the street in the back (to the right side of *her* head, our left) provides more lighting for the AF to lock on. Had you been in Spot-AF, I think the camera would have had a better chance, but AF would have been slowed down to a hunt.

In this situation... I would have tried to locked on to her left arm, where it meets the guy's shirt. There is contrast there... just as much as the face/hair, but at least it's low and will not have anything in the background to distract it. Or even the girl to camera's left, with the white long sleeve shirt. But again... it could be that we see contrast simply because the flash fired and lit up the subjects... whether the focus assist lamp was able to provide sufficient coverage or not... that's yet to be determined.

The FA lamp doesn't shoot all that wide (side-to-side, up-n-down).

If I was shooting that shot using manual point selection, I would have positioned the focusing point lower in the scene, probably onto the baby's chest or the arm/shirt interface as Julian suggested. I would not use spot focusing, but regular single point focusing because when positioning the focusing point on the bodies that are positioned in approximately the same focal plane, a larger focusing point would stand a greater chance of locking onto something with contrast rather than a smaller focus point.

Another alternative you might have tried was using full automatic point selection (e.g., the big brackets). This focus mode will use all 19 focus points and lock focus on whatever subject is closest to the camera that has good detail/contrast. In your shot, the closest subject is probably the man's face, so you might have gotten good focus using that mode. Zone AF where the zone is positioned at the bottom may also have worked (this focus mode will lock focus on whatever subject is closest to the camera in the AF zone that has good detail -- it's like a smaller version of the automatic selection AF mode).

Julian thanks for the advise. I will surely try that next time im in that situation. I should have just focus on something better then recompose. It could have given me a better shot. I will practice more on focus then recompose. DOF should have been deep enough for any margin of error.

tgara, that's actually a great advise! I totally forgot that if I had used zone AF, it would have locked on to whatever is closer! If you had not mention it, it would still elude me as a better option!

I guess this is where inexperience and not knowing the gear 100% (or knowing but forgetting to use when it could have helped me) kicks me in the butt. But im still learning!

musashi wrote in post #11564247
Julian thanks for the advise. I will surely try that next time im in that situation. I should have just focus on something better then recompose. It could have given me a better shot. I will practice more on focus then recompose. DOF should have been deep enough for any margin of error.

tgara, that's actually a great advise! I totally forgot that if I had used zone AF, it would have locked on to whatever is closer! If you had not mention it, it would still elude me as a better option!

I guess this is where inexperience and not knowing the gear 100% (or knowing but forgetting to use when it could have helped me) kicks me in the butt. But im still learning!

The only problem with the zone approach is that it still could lock focus on something behind your intended subject, as that "something" could be the closest thing it "sees" with good contrast to lock focus on.

Yes that's true, but not if the focus point + assist point all covers people on the group. Im not saying it will 100% accurate, but it will give me better chance of nailing focus on any of the 7 subjects in front of me, not some light or building 50 ft behind the intended target.

If it still locked focus somewhere else, well then I gues im SOL. lol.

musashi wrote in post #11564859
Yes that's true, but not if the focus point + assist point all covers people on the group. Im not saying it will 100% accurate, but it will give me better chance of nailing focus on any of the 7 subjects in front of me, not some light or building 50 ft behind the intended target.

If it still locked focus somewhere else, well then I gues im SOL. lol.

Let us know how it works - inquiring minds want to know.

I have once taken a picture with EXACTLY the same result. I had the focus point on the subjects face, but the camera selected to focus on something halfway into the next focus point, because there was a window, and outside it was brighter than inside. So the camera locked on the window frame and the person I was shooting got out of focus.

So this is one of the characteristics of the 7D. Not much to do about it, but learn to live with it. Aim somewhere where the camera can't see another nice target (in the picture in this thread it obviously noticed the nice contrast the illuminaged signs in the background provided).

Can someone enlighten me. I only have a 20D. If you choose the focus point you want to use, how does the camera override that. Never had that happen.

you said that you took 2-3 shots of the same pose...do you have a shot of the group in focus? In this situation I would have tried to do slight MF to achieve proper focus.

ps. I would second the advise of separating AF to the back button. Weird but my cameras seem to focus more consistently with back button AF...

kjonnnn wrote in post #11565044
Can someone enlighten me. I only have a 20D. If you choose the focus point you want to use, how does the camera override that. Never had that happen.

It's more that the selected focus point focuses on something outside of the focus square/rectangle in the viewfinder. This is possible because the focus sensor covers a significantly larger area in the scene than the portion of the scene covered by the focus square/rectangle in the viewfinder.

kjonnnn wrote in post #11565044
Can someone enlighten me. I only have a 20D. If you choose the focus point you want to use, how does the camera override that. Never had that happen.

That's because the 7D has a transmissive LCD screen that displays the AF point, and not actually the AF point itself. So the screen gives you a rectangle signifying the selected AF point. But it's not a perfect representation of the actual size of the sensor in the region.

It's one of those "learn-to-deal-with" features that once you understand... it's not really an issue at all.

stsva wrote in post #11564631
The only problem with the zone approach is that it still could lock focus on something behind your intended subject, as that "something" could be the closest thing it "sees" with good contrast to lock focus on.

That would not happen in the OP's shot because setting the Zone AF to the bottom center position necessarily makes it read off a "wall" of bodies. There is no way the AF points could see anything behind that.