AF question (a long story)

[Outdoorsman]

Let me preface this by saying that I have absolutely no clue as to how autofocus works.
I use an Elan 7N, with 7 AF points. This last weekend I was down at our aptly named Beachfront Park and I saw a hawk sitting right next to the waterfront foot/bike path. Just sitting there within 10 feet of cars. I was able to get about 10-20 feet from it before it flew down the path and parked itself again. It was totally unafraid.
I was using my camera's AI Servo AF mode which is like a cross between One Shot and Servo. It locks onto stationary objects but adjusts when it senses the lens-to-subject distance has changed.
I was framing vertically from below the bird with the proper AF point selected to get the bird more or less centered in the frame while focusing very carfully on the eye. When I saw that nice "catch-light" in the eye I took the shot. But several times during this "shoot" the AI Servo decided that the stationary bird was no longer in focus and adjusted to focus on some other thing (I don't know what) that was ever so slightly not the eye. The eye of the bird did not occupy the whole of the AF point and the bird did not move.
Now, the AF points I have (I'm not familiar with other cameras) are little rectangles.
My question: is there a specific part of this little rectangle that I should place my subject in for more accurate/reliable AF?
I'm just trying to figure out why the AF screwed up in this case...

[paulnj]

Sounds like a job for ONE SHOT, CLOSEST SUBJECT PRIORITY and a SINGLE AF point (not all 7 active) If some of the feature don't apply to the Elan 7 , sorry

[another view]

First off, I shoot Nikon so the terminology is different - and the modes available might be different. But I think I'd agree with bird-master Paul on "Single" AF mode. I find (again, at least with Nikons) that the other AF mode (continuous) is great for somthing that you'd have an otherwise difficult time getting a good shot of. It's just not precise enough to know that the eye is what you want in focus, so you may want to stop down a little more for more DOF (margin of error).

I know this isn't always a good option - the background can become busy and the shutter speed can be too slow. Can you make manual adjustments to the focus when the lens is in an AF mode? It might be very helpful for fine-tuning.

[paulnj]

AI servo stands for artificial intelligence basicly he was in continuous AF that was always searching to AF on what the camera felt was the subject to focus on. closest subject priority is good for flight shots and clear foreground / background shots. to me it sounds like he was shooting on an upward angle with mostly sky as the BG.

Canon and Nikon are very similiar in features and how they work for the most part. I shoot my F100 the same as I shoot my 1D.

[Lemming51]

AI Focus is supposed to be a combination of One Shot and AI Servo, but in my experience is not as good as either. It won't hold focus as well as One Shot and is much much worse at tracking focus than AI Servo. I never use it anymore and simply select One Shot or AI Servo depending on the situation.

I think it's instructive to note that the AI Focus mode is found only on the Rebel and Elan lines and not on the EOS 3 or 1 bodies. ;-)

[Clemmie]

As one who must deal with 9 AF sensors (Maxxum 7), trust me: There are times when you need to preselect just one sensor - and this is one of those times.

[paulnj]

Ah ha ! Since I shoot a EOS 1 based camera, I just figured he was using AI servo ;) I learned something today

[Outdoorsman]

Thanks so much for all the responses. I really appreciate your help.
But....
Re-read my original post: "with the proper AF point selected"....
There are five points spaced horizontally and three vertically (counting the center point twice). I was shooting vertically, so this makes the point I had selected the one (in the horizontal orientation) to the immediate right of the center point.
So since I did choose just one AF point, that is not the issue. I normally shoot with AI Servo since it usually works very much like One Shot-- the AI function seems to be sluggish in responding usually, which is fine, since I'm used to just releasing the button and re-focusing again very quickly. I just don't see a reason in this case for it to suddenly decide that the subject had moved. It's not terribly adept at picking up moving objects in AI Servo.
The AF points are little rectangles in my viewfinder.
My Question: is there a specific part of this little rectangle that I should place my subject in for more accurate/reliable AF?
Thanks again everybody!

[another view]

My Question: is there a specific part of this little rectangle that I should place my subject in for more accurate/reliable AF?

Probably not going to make a difference. Note that the actual area measured might not exactly correspond with that little rectangle etched into the focusing screen. It may be a little larger than that rectangle, and/or to the left or right, etc. The thing is, it's focusing on something in that area but it may or may not be the subject's eye. It will get you very close in a situation like this but it's never going to be 100%. This is why I say that stopping down can buy you a little "insurance" with more DOF.

[92135011]

and THATS why I stick with fully manual cams
I dont trust them silicon chips