Canon EOS-1D X Manual (en)

You can make corrections to any of the five aberrations as below, either 
one at a time or simultaneously.
Peripheral illumination 
Chromatic aberration (color fringing which occurs in the periphery of 
an image) 
Color blur (blue or red color blur which sometimes occurs at the edge 
of the highlighted area of an image) 
Select an image in the main window.
Display the Lens Correction tool palette and 
checkmark the items to be corrected.
The image is corrected and redisplayed.
In the main window, the [
mark is displayed on the image to 
indicate that the lens aberration correction is applied 
Making Corrections
Enlargement display section
Check the results of corrections to peripheral illumination or 
distortions in the full view.
Check the results of corrections to chromatic aberration and color 
blur by clicking the area on the image you want to check and 
then checking the enlargement display section in the top of the 
window. You can choose from 4 display scales between 1:1 and 
1:4 for the enlargement display.
You can lessen the degree of correction by moving the slider to 
the left.
You can respectively correct red chromatic aberration with the [R] 
slider of the [Chromatic aberration] or blue chromatic aberration 
with the [B] slider.
To correct distortion aberration of images shot with a fisheye lens 
(EF 8
15mm f/4L USM Fisheye or EF 15mm f/2.8 Fisheye), four 
effects can be selected from the [Effect] list box in addition to 
[Shot settings] normally used for distortion correction.
The level of an effect can be adjusted with the adjustment slider 
located below [Distortion].
For detailed information on each effect, see “Effects of Distortion  
Correction for Fisheye Lenses” 
For images shot with EF 8
15mm f/4L USM Fisheye, a fine-tune 
slider is also displayed when you select [Emphasize Linearity] 
from the [Effect] list box. 
Adjust the amount of correction for distortion aberration with the 
adjustment slider, and then adjust with the fine-tune slider if more 
adjustment is required.
When the list box for selecting an extender is displayed, select 
the extender attached at the time of shooting from the list box.
[Diffraction correction] corrects only those images shot with a 
camera with a diffraction correction function. By checkmarking 
this option, images are corrected according to the camera’s 
diffraction correction  function. Note that this function cannot be 
used at the same time as the Digital Lens Optimizer.