Canon EOS-1D X Mark III Advanced User's Manual page 159

Digital Lens Optimizer
Various aberrations from lens optical characteristics can be corrected, along with diffraction
and low-pass filter-induced loss of resolution.
If [Correction data not available] or [
use EOS Utility to add the lens correction data to the camera. For details, refer to the EOS
Utility Instruction Manual.
Caution
Depending on shooting conditions, noise may be intensified together with the
effects of correction. Image edges may also be emphasized. Adjust Picture Style
sharpness or set [Digital Lens Optimizer] to [Disable] as needed before shooting.
The higher the ISO speed, the lower the amount of correction will be.
For movie recording, [Digital Lens Optimizer] will not appear. (Correction is not
possible.)
Note
With [Digital Lens Optimizer] set to [Enable(Std)], [Chromatic aberr corr] and
[Diffraction correction] are not displayed, but they are both set to [Enable] for
shooting.
] is displayed by [Digital Lens Optimizer], you can
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