Canon EOS-1D X Mark III DSLR Camera (Body Only) - Manual (Page 227)

Evaluative metering
General-purpose metering mode suited even for backlit subjects. The camera adjusts the
exposure automatically to suit the scene.
Partial metering
Effective where there are much brighter lights around the subject due to backlight, etc.
Spot metering
Effective when metering a specific part of the subject. The spot metering circle will be
displayed in the viewfinder.
Center-weighted average
Metering over the entire area is averaged, with the center of the viewfinder weighted more
heavily.
Note
By default, the camera will set the exposure as follows.
With <
>, holding down the shutter button halfway locks the exposure (AE lock)
after the camera focuses with One-Shot AF. With < > / < > / < >,
exposure is set at the moment the picture is taken (without locking the exposure
value when the shutter button is pressed halfway).
You can link spot metering to all AF points by setting [
1 : Spot meter. linked to
AF pt ] to [ Linked to active AF point ] ( ).
With [ 2 : AE lock meter. mode after focus ] ( ), you can set whether or not to
lock the exposure (AE lock) when focus is achieved with One-Shot AF.
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