AF mode questions *one shot" VS "AI servo"

[praline3001]

I am reading reading reading! At this point I feel I know just enough to be dangerous!!!!!

I have been studying the AF mode dial stuffs on my canon T3i. The book I am currently reading has study assignments and reviews ~ almost like a text book? Anyway, they want me to set it on manual selection and the center square only as the AF point. Then they want me to keep *one shot* for the AF mode.

Is there a difference in the focus sharpness between one shot and AI SERVO? I know what AI SERVO does, the focus moves with the subject. I use this a lot shooting my kids and pets (with my camera not my rifle hehe).

Why is there a need for *one shot* when AI SERVO will focus on still and moving objects? Is one shot a crisper image? To me, it seems like *one shot* is worthless unless the it gives a sharper image then using AI SERVO?

Can someone explain this to me PLEASE...

The book I am reading is, " Canon EOS Rebel T3i/600D from snapshots to Great Shots" by Jeff Revell.

[RichardTaylor]

With one shot you can focus and recompose. AI servo you can't.

[Doug Pardee]

With One-Shot you can use multiple focus points and you can focus-lock. Both of those allow you to take pictures where the subject isn't centered in the frame. In general, centered subjects are less interesting.

The following are about Canon, and may not apply to other makes:

Using One-Shot along with Evaluative metering means that exposure will lock along with AF when you half-press. You don't need to use the * button to get AE Lock. (Exposure lock obviously doesn't apply to M mode.) This is another factor that simplifies taking photos with subjects that aren't centered.

Evaluative metering isn't as effective at giving a "good" exposure if you aren't using One-Shot or if you don't have all AF points enabled. It's more easily fooled by large subjects that are unusually bright or dark.

One-Shot will wait for focus to be achieved before releasing the shutter, and will keep you from taking a picture when focus can't be achieved. If you're using AI Servo, the camera assumes that you're taking an action shot and that time is of the essence — the shutter is immediately released, focused or not. For a continuous burst, this only applies to the first snap; the rest wait for AF to be achieved.

One-Shot will light up the red LED(s) that have achieved focus lock, and will sound the beeper if you have it enabled. AI Servo does neither of those things.

Overall, One-Shot is extremely handy for snap-shooting, but not so much for the serious photographer. Many of us use back-button autofocus along with AI Servo (and center-point autofocus and a metering mode other than Evaluative) to give on-the-fly choice between focus lock and continuous focus. With a Full-Time Manual lens, we also get on-the-fly choice of manual focus. Using AI Servo also means no AF delay on shutter release, although we must be careful that we've already achieved focus. This setup puts a noticeably higher workload on the photographer, but is considerably more flexible.

[praline3001]

Thanks for such a great explanation Doug!

I am a reasonably smart person but this techie camera stuff boggles my mind!!

I had my AI SERVO on constantly. I didn't see the real need for the "one shot". Now I do.

The book I am reading explains that I should use eval metering with manual AF mode and only using the center square while learning. I can move the center AF square using my AF button. I worked a bit last night with *in house* subjects such as pencils, laundry soap and cookies. I was able to get sharper off center photos, more following the rule of thirds.

The book has me doing this while learning how to run everything else on *M*. It does explain how to manual focus and why the camera may take a billion years to focus on something. I have been using Manual exposure (ISO,AV,TV) but only have manual focused a few times.

I am trying to stick with the steps and assignments in the book but was having issues with using *one shot* rather then AF SERVO. As the lessons for this particular chapter are about recomposing shots and focusing on something not in the center for the rule of thirds... I can see why they want me to go with it ~ it makes sense now