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First I may be using the term "continuous auto focus" in the wrong context but i don't have my book with me. I know there is a setting on my 550D that allows contiuous focus on a moving subject. The way I under stand it is once I half push the shutter and it catches focus, if the subject moves it will keep it in focus. So my first question is what is it actually called, is it continuous focus? Second, where in the menu do i change it (if i knew the name i could prob find my self. Lastly, this is the main reason i ask. I just got my first 300mm. This lens doe not ingauge the "IS" until you half press the shutter, so the point of focus may not be right on the subject. What i'm trying to find out is if i keep the shutter reliease half pressed them move to the subject will the focus adjust automaticly?

Sorry if this rambles a bit, i'm typing on my cell

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Mike "The Squirrel"
Canon 550D | Canon EF 35mm 1:2 | Canon 50 f/1.8 II | Sigma 18-125mm DC OS | Tamron SP 70-300mm Di VC USD | Canon 430EX II
Military Issued Canon 40D | Canon 55-250mm IS

Continuous focus is the generic name for it, although Canon call it "AI Servo".

On the 600D the AF selection is entered by pressing the right-direction button on the back.

Note you can use it with single or all AF points active, set separately. With all points active it would tend towards the closest object, unless it is small or much lower contrast than something further away. With single point, it will of course be whatever is under that point. If this isn't right, you can momentarily un-press then half-press the shutter again to force another lock attempt.

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Canon DSLRs: 7D, 5D2, 1D, 600D, 450D full spectrum, 300D IR mod
Lenses: EF 35/2, 50/1.8, 85/1.8, 135/2+SF, 28-80 V, 70-300L, 100-400L, TS-E 24/3.5L, MP-E 65, EF-S 15-85 IS
3rd party: Zeiss 2/50 makro, Samyang 8mm fisheye, Sigma 150 macro, 120-300 f/2.8 OS, Celestron 1325/13
Tinies: Sony HX9V.

Thanks! So lets say i'm trying to focus on a horse far off so when the stabilization kicks in i realize that is focusing on his flank, if i held the half press on the shutter and moved to its face where i want to focus, will the ai servo adjust the focus? I'm finding i'm so unsteady without the stabilization at 300mm that when it kicks in I'm not on the point of focus i want so if i let go and try again i still miss the point i want.

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Mike "The Squirrel"
Canon 550D | Canon EF 35mm 1:2 | Canon 50 f/1.8 II | Sigma 18-125mm DC OS | Tamron SP 70-300mm Di VC USD | Canon 430EX II
Military Issued Canon 40D | Canon 55-250mm IS

If you're using a single point AF, it will focus on what's under the point. If you're using all points, it could be anything.

If you momentarily let go of the shutter button AF should continue for some seconds after.

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Canon DSLRs: 7D, 5D2, 1D, 600D, 450D full spectrum, 300D IR mod
Lenses: EF 35/2, 50/1.8, 85/1.8, 135/2+SF, 28-80 V, 70-300L, 100-400L, TS-E 24/3.5L, MP-E 65, EF-S 15-85 IS
3rd party: Zeiss 2/50 makro, Samyang 8mm fisheye, Sigma 150 macro, 120-300 f/2.8 OS, Celestron 1325/13
Tinies: Sony HX9V.

This is where removing the AF function from the shutter and moving it to the AE-lock (*) button is soooo much better. You can keep the shutter half-pressed to start metering and IS, and then focus using the AE-lock button. You can let go of the (*) to "lock focus" and maintain that focus as well, firing away as necessary with the shutter. This method is called "back-button focusing" and IMHO it is far and away superior to shutter-focusing... unless you're shooting a lot of stuff with exposure lock.

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Canon EOS 5D MkII | Canon EOS 7D | Canon Digital Rebel XSi | EF 35mm f/1.4L | EF 50mm f/1.8 II | EF 85mm f/1.8 | EF 100mm f/2.8 Macro USM | EF 135mm f/2L | EF-S 18-55mm f/3.5-5.6 IS | 580EX II | LumoPro LP-120

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