Canon EOS-1D X Mark III Advanced User's Manual page 73

Adjusting the Viewfinder
1.
Detach the eyecup.
Grasp the eyecup on both sides and slide it up to detach it.
2.
Make the adjustment.
Turn the knob left or right to make the AF points in the viewfinder look
sharp.
Attach the eyecup.
Note
If the camera's dioptric adjustment still cannot provide a sharp viewfinder image,
using Dioptric Adjustment Lenses Eg series (sold separately) is recommended.
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