Canon EOS-1D X Mark II DSLR Camera (Body Only) - Manual (Page 197)

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3 Correction of Lens Aberrations
The lens correction data for lens aberration corrections is registered (stored)
in the camera. With [
Enable ] selected, the peripheral illumination, chromatic
aberration, distortion, and diffraction will be corrected automatically.
With EOS Utility (EOS software, p.552), you can check which lenses
have their correction data registered in the camera. You can also
register the correction data for unregistered lenses. For details, refer to
the EOS Utility Instruction Manual (p.554).
For lenses incorporating the correction data, it is not necessary to
register the correction data to the camera.
Lens Correction Data
Cautions for Lens Correction
Peripheral illumination correction, chromatic aberration correction, distortion correction,
and diffraction correction cannot be applied to JPEG images already taken.
When using a non-Canon lens, setting the corrections to [ Disable ] is
recommended even if [ Correction data available ] is displayed.
If you use magnified view during Live View shooting, the peripheral
illumination correction, chromatic aberration correction, and distortion
correction will not be reflected in the image on the screen. Note that the
diffraction correction will not be applied to the Live View shooting image.
The correction amount will be less (except for diffraction correction) if the
lens used does not have distance information.
Notes for Lens Correction
The effect of the lens aberration correction will vary depending on the
lens used and shooting conditions. The effect also may be difficult to
discern depending on the lens used, shooting conditions, etc.
If the effect of the correction is not visible, magnify the image after
shooting and check it again.
Corrections can be applied even when an Extender or Life-size Converter is attached.
If the correction data for the attached lens is not registered to the
camera, the result will be the same as when the correction is set to
[
Disable ] (except for diffraction correction).
See pages 397 and 398 regarding Digital Lens Optimizer’s correction data
for when shooting RAW images.