One shot means it focuses once when you half depress the shutter button, it'll stay that way until you release the shutter button...
AI focus switches between the on the cameras plan, it starts on one shot and will change if the camera detects motion between frames, it used with burst mode, though it isn't great on a lot of canon models.
AI servo constantly adjusts focus until you fire the shutter, you focus on an object and the focus will "chase" it...
One shot AF is meant for stationary subjects that are not about to move. Great for still life, landscapes, macro, portraits, architecture and interiors. Ideal for everyday shooting as you can't take photo until subject is in focus.
AI Servo AF is for action subjects that are constantly moving. It continually tracks movement and refocuses for them. You can full press shutter with this mode on as it will then focus before shooting.
AI Focus AF is for stationary subjects that may move. It basically switches between the other two modes as it sees a need.
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AI focus switches between the on the cameras plan, it starts on one shot and will change if the camera detects motion between frames, it used with burst mode, though it isn't great on a lot of canon models.
AI servo constantly adjusts focus until you fire the shutter, you focus on an object and the focus will "chase" it...
AI Servo AF is for action subjects that are constantly moving. It continually tracks movement and refocuses for them. You can full press shutter with this mode on as it will then focus before shooting.
AI Focus AF is for stationary subjects that may move. It basically switches between the other two modes as it sees a need.
Hope this helps.