The x0D Canon line all offer this option. In use, I could never quite figure out why someone would choose AI Focus rather than the AI Servo option, unless it's to save a little battery use.
Then I purchased a Mk III, and found only two focus options - One Shot and AI Servo. Less decisions.
So why does Canon think that their prosumer line needs the AI Focus option, and not their pro line?
Does anyone ever choose AI Focus? If so, why? Or am I being dim and missing the point about some finer point of AI Focus? I do understand that AI Focus initially locks, but tracks movement if the subject starts to move, but it still doesn't allow you to recompose on a still subject. On the Mk III one can momentarily disable AI Servo with a button push, to recompose; I don't think the x0D line has this ability.
Any comments would be appreciated - though preferably not confirming that I'm dim!
I never use AI focus, and I rarely use One Shot. On both my Mark III and my 50D, I have AF set to the middle button on the back of the body. The shutter button initiates metering and also takes the photo. That way I am always in AI Servo; all I do is press and hold the focus button to initiate and maintain AF. If I want to focus and recompose, I press the focus button to lock AF on the subject and then I release the focus button. Now I can recompose. When I press the shutter button to take the photo, the camera does not refocus; it simply takes the recomposed photo.
I've read the manuals, but somehow hadn't fully realised the significance of this Custom control. And I much prefer the act of pushing a button to make something happen (i.e. auto-focus) rather than having to take your finger off a button to stop an action. And for re-composing in AI Servo mode, it is much more convenient.
Now I just have to practice a bit, so that it becomes second nature to me.
Some people cannot get the coordination down of using the back button to AF. If this holds true for you depress and hold the AF-L BUTTON with your thumb to lock focus, recompose, tweak focus manually, etc. Lift your thumb and the camera will again focus in AIS.
FYI- on static subjects One-Shot will offer more precise and consistent sharpness, as the AF will not oscillate as when in AIS. It is often necessary to pump the focus to get it sharpest. This is visible in the viewfinder. The AF tolerance, even with mico-adjustment is a bit sloppy, and disturbing to say the least.
AI Focus and AI Servo
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