How to Lock Focus on Your Canon EOS 6D
Your EOS 6D offers you two ways to lock focus on an object that isn't in the center of the frame: the shutter button or the AF-On button. This option comes in handy when your center of interest isn't in the center of the frame.
To lock focus with the shutter button:
While looking through the viewfinder, move your camera until the center of the viewfinder is over the subject that you want the camera to lock focus on.
Press the Shutter button halfway.
Make sure that a black autofocus square appears over your subject. You can switch to a single autofocus point that's in the center of the frame and center the single autofocus point you’re your subject before locking focus. When the camera achieves focus, a green dot appears in the viewfinder.
While holding the Shutter button halfway, recompose your picture.
Press the Shutter button fully to take the picture.
You can also lock focus with the AF-On button on the back right-hand side of your camera near the top. This is a little easier because you don’t have to hold the Shutter button halfway while composing your picture. To lock focus with the AF-On button:
Look through the viewfinder and move the camera until the center of the viewfinder is over the subject that you want the camera to lock focus on.
Press the AF-On button.
Red autofocus squares appear momentarily in the viewfinder over the subjects that the camera will lock focus on. The squares turn black after the camera achieves focus. Make sure the autofocus points that display are over the object that you want the camera to lock focus on.
Move the camera to recompose the picture and then press the Shutter button fully.
An image appears almost instantaneously on the camera LCD monitor. Review the image to make sure the camera locked focus on the desired object.