Halp Ples!

[Matblack]

I am struggling a bit to find information on my cameras autofocus settings, its a canon 300d and as far as I can see it has three settings

OS - One shot? - In this mode I can select a focus point or multiple focus points and get the camera to lock to one of these by depressing the shutter halfway, once I have it held there I can move the camera and the focus will be held.

But I have two other settings

Ai - This seems to do the same thing as OS

SE - I call this one Elmer Fudd mode because it does a lot of hunting but never pulls the trigger, it just refuses to pop up that focus lock icon and stops the camera firing when I depress the shutter all the way.

With all of these I can move the focus points around which seems useful, I suspect if I want to take a landscape with some one in front of it I could move the focus point to the point they are standing and get the camera to focus there, not sure why I wouldn't use the halfway focus lock and move to create the composition but I think thats what it does.

Please can someone explain the difference between SE and OS and what Elmer Fudd mode does

Thanks

MB