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Old Mar 9, 2004, 3:14 PM   #11
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barthold

In my experience, that is the case. My guess (I'm not sure & I don't have the manual) is that the subject in the center will be what it locks the focus on to, and then after that is uses all 7 to track and keep in focus. I assume if I had the left most selected, it would use that one to find the subject to track, but then use all to track as necessary.

GregDunn
What you describe is how it's supposed to work... which is why (like you) single point AF is the way to go.

Eric
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Old Mar 9, 2004, 9:10 PM   #12
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99.9% of the time, the remainder being tests that didn't work out very well.
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Old Mar 9, 2004, 9:31 PM   #13
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Eric, ok that would be pretty cool if it works that way! Norm, do you use AI-servo for your basketball shoots ? Wouldn't that help you track the players running around?

Barthold
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Old Mar 9, 2004, 9:54 PM   #14
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Barthold...

Your suggestion got me doing a bit of reading and I came up with this interesting piece of info:

http://www.xmldatabases.org/WK/blog/..._AI_Servo.item

As much as I hate using the preset modes I'll try your suggestion and see what happens but I usually shoot exclusively in "M" mode, which may be a mistake in this particular instance "if" the Sports mode actually works for me.

As for the "hack" mentioned in the above reading, nope. I'm not that brave.
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Old Mar 9, 2004, 9:58 PM   #15
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Man I didn't realize that you can have a set af point in servo mode. This will change how I use my camera for certian things! Maybe I should have read that silly book that came with my camera. What are those called again? :lol:
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Old Mar 9, 2004, 10:20 PM   #16
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http://www.shortcourses.com/10D/focu...s%20Prediction

AI Servo AF
AI servo mode continually adjusts the focus as long as you hold the shutter button halfway down. Itís designed to help you keep a moving subject in focus and is great for sports and nature photography, or any other situations where you are photographing fast-moving subjects. As you pan to follow the subject, the autofocus system follows the subject on the active focus point and keeps it in focus. The camera uses your selected AF point to focus unless you are using auto, in which case it uses the center AF point. If the subject then moves, it will remain in focus as long as itís covered by one of the seven focus points although the one being used isnít displayed in red. Even when focus is achieved in this mode the focus confirmation light doesnít light and the beeper doesnít sound. AI servo is the autofocus mode selected for you in Sports mode. When a subject is moving toward or away from the camera at a fixed rate, the camera can predict where it will be when the shutter opens and ensure that itís in focus. In this modeócalled predictive autofocus, the camera will shoot even when a subject isnít in focus and exposure is determined just before the picture is taken.
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Old Mar 9, 2004, 10:56 PM   #17
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I'm a centre AF point and recompose person too. The only time that I do things differently is when I'm shooting on a tripod and I have the time to move to an off-centre spot.

One of the things I'm hoping that the eventual 10D replacement will have is eye-controlled focus like the EOS 3 or Elan 7E.

Is there a reason why Canon hasn't implemented eye controlled focus points on their digicams? To my knowledge none of Canon's dSLRs offer it.
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Old Mar 10, 2004, 12:52 AM   #18
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Eric, OHenry,

From reading the 10D manual I interpret it that if you have selected a single AF point, then the predictive AF focus will only use that point. Thus it will not switch to use the 7 points. Predictive AF focus will only use the 7 points if you have all 7 AF points active (called automatic AF point selection). See page 66 of the 10D manual.

The text in red Ohenry quoted can be read to mean the same thing, although it isn't written very clearly.

Barthold
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Old Mar 10, 2004, 8:47 AM   #19
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Quote:
Originally Posted by barthold
Eric, OHenry,

From reading the 10D manual I interpret it that if you have selected a single AF point, then the predictive AF focus will only use that point. Thus it will not switch to use the 7 points. Predictive AF focus will only use the 7 points if you have all 7 AF points active (called automatic AF point selection). See page 66 of the 10D manual.

The text in red Ohenry quoted can be read to mean the same thing, although it isn't written very clearly.

Barthold
That's the way I read it too, Barthold. (The text I quoted is written by the people that write the Short Course books and was related to the 10D. I assume the same applies for the Rebel)
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Old Mar 10, 2004, 10:15 AM   #20
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One thing that is kind of vague in the manual is the description of how AI Servo actually behaves. I've read several messages from people who are confused by the text and by the actual operation of the camera. I have some experience with servo-controlled systems, so I undertook an experiment.

I hypothesized that the camera gets its data during the period from the time you half-press the shutter until it acquires the subject indicated by the AF point(s); as soon as it decides focus has been achieved, it chooses a mode: either single-shot or Servo.

After some hand-waving (literally -- I used my hand as an AF target as well as some more sophisticated tests with small moving objects) it seems that the camera samples the AF point(s) during the focus acquire period and if it detects motion DURING THAT TIME, puts the camera in AI Servo. Motion == change in measured focal distance. If the subject is stationary while the camera is obtaining focus info, once it locks it will not switch to Servo mode. I tried the same things that forced a switch to Servo, after I had carefully locked focus on a still subject, and it resolutely stayed at my chosen focus distance.

I tried everything I could think of to confuse the camera during the tests, and the above description still seems to characterize its behavior. I was able to kick it into Servo mode reliably every time, or force it to lock into single-shot. I was using sngle-point AF for more control over the test conditions. Now, if you're trying to focus on slow-moving or low-contrast targets, I can understand the camera not "seeing" subject motion and staying in single-shot. It needs to measure a change in subject distance during that acquire period. Likewise, in low light or if you use the 7-point focus your focus acquire time is going to be longer and/or a change in focus "data" more noticeable, so the camera is more likely to switch to Servo on questionable motion. But it does seem deterministic and repeatable as far as I can tell -- as long as you keep in mind what it's doing to evaluate the subject's "status".

Does this seem right?
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