# AF mode question



## Mygixxer (Mar 31, 2012)

shooting with a canon T2i. looking for some clarification on when to use what AF mode. I saw a couple threads on one shot vs. AI servo.

between Single shot, AI focus and AI Servo, what circumstances would call for which?


----------



## belial (Mar 31, 2012)

Single shot is for non moving objects. For example a landscape. Ai servo is for movin objects for example sports. And ai focus is when you think you may encounter a variety of moth of the above circumstances as the Camera automatically switches between both modes for you


----------



## analog.universe (Mar 31, 2012)

One shot focuses once, and then locks focus for as long as you hold the button.  In order to change the focus, you need to press the button again.  Additionally, you can not release the shutter until focus has been locked.  This is useful if you want recompose the shot after you've focused (although care should be taken with this method, especially on wide lenses).  AI servo continually refocuses for as long as the button is held down, and allows you to release the shutter at any time.  AI focus supposedly decides for you, be to me it's just unpredictable...


----------



## Mygixxer (Mar 31, 2012)

ok that helps! thanks guys


----------



## TCampbell (Mar 31, 2012)

There are two other nuances... 

In "One Shot" mode the camera uses "Focus priority".  Meaning if the lens supports AF and the AF is enabled, then when you fully-press the shutter button the camera won't take the shot in that mode until it can confirm that at least one focus point locked focus.  

In "Ai Servo" mode the camera uses "Release priority".  Meaning when you fully press the shutter button, the camera will take the shot immediately... whether AF was able to get a focus lock or not.

There's a way to move focus to the a back-button on your camera (I can look it up if you want to know how it's done... on my camera it's the first option under Custom function IV.)  It was put there for sports photographers and here's the idea.  Suppose you're shooting a football game from the sidelines.  You're tracking the action down the field, but there are other people on the sidelines.  You're shooting away when all of a sudden you realize that someone else on the sidelines has is in the way of the action (and you're panning the lens to follow the action down the field.)  What you do NOT want to have happen is for the camera to re-focus on the close person.  The Canon AF system always selects the AF point that can find the NEAREST subject to lock focus on.  That means it will _always_ re-focus to the close person on the sidelines rather than the players out on the field (pretty much not what you want.)  

AI Servo lets you ignore the person in the foreground (if the camera thinks it's not focused it doesn't matter... it takes the shot anyway) and by moving focus to the back button you can independently meter and fire with the front button and while focusing with the back button.  As a person enters the edge of the frame, you just let off the focus button and keep shooting.


----------

