Canon EOS 90D Advanced User's Manual page 159

By default, the camera will set the exposure as follows.
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With < q >, holding down the shutter button halfway will lock the exposure
setting (AE lock) after focus is achieved with One-Shot AF. In the < w >, < r >,
and < e > modes, the exposure is set at the moment the picture is taken.
(Pressing the shutter button halfway does not lock the exposure.)
With [ 8 C.Fn I-8: AE lock meter. mode after focus], you can set whether or
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not to lock the exposure (AE lock) when focus is achieved with One-Shot AF
( = 558).
Selecting the Metering Mode
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