Canon EOS R5 Mark II Advanced User's Manual page 256

Diffraction Correction
Diffraction (loss of sharpness caused by the aperture) can be corrected.
Caution
Depending on shooting conditions, noise may be intensified together with the
effects of correction.
The higher the ISO speed, the lower the amount of correction will be.
Note
"Diffraction correction" corrects degraded resolution not only from diffraction but
also from the low-pass filter and other factors. Thus, correction is also effective for
exposures with the aperture wide open.
[Diffraction correction] is not displayed when [Digital Lens Optimizer] is set to
[Standard] or [High].
Caution
General precautions for lens aberration correction
Lens aberration correction cannot be applied to existing JPEG/HEIF images.
When using a non-Canon lens, setting the corrections to [Disable] is
recommended even if [Correction data available] is displayed.
Magnifying the periphery of the image may display parts of the image that will not
be recorded.
The amount of correction (except diffraction correction) is less for lenses that do
not provide distance information.
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