EOS 5D Mark IV Diffraction Correction
As part of the lens correction options, the EOS 5D Mark IV features a Diffraction Correction option to remove any diffraction caused by the lens while shooting. This setting works well and helps ensure you achieve the best possible results from your lens. Under some shooting conditions though, there may be increased noise shown in the image, especially in the shadow areas. If this happens, either reduce the ISO speed where possible, or disable Diffraction Correction and instead apply it using the Canon Digital Photo Professional software on your computer.
Source Canon Professional Services.
Contributed by: Martin J. Allred