Canon EOS-1D X Manual (en)

Emphasize Center
This converts the image to an “orthographic projection” style image. 
With this projection method, the center is more emphasized.
Since an orthogonal projection allows a subject of the same 
brightness to be displayed as occupying the same area in the image, 
this method has typical uses in photography for celestial luminance 
distributions and center-emphasized animal photography. By 
selecting this option, the center is stretched and may result in 
decreased resolution.
When selecting any of the options, [Emphasize Linearity], 
[Emphasize Distance], [Emphasize Periphery], or [Emphasize 
Center],  it will yield results similar to that of the selected projection 
method. However, the conversion does not really apply a change of 
projection method in a strict sense.
Accordingly, be aware that they are not suited for scientific or other 
such purposes.
When correcting RAW images and if shooting distance information has 
been saved to the image, the slider is set automatically according to 
this shooting distance information. However, if no shooting distance 
information is saved to the image, the slider is set automatically to the 
right end at infinity position and a [<!>] appears above the slider. 
If the image’s shooting distance is not infinite, you can minutely adjust 
the distance by operating the slider while looking at the screen.
For RAW images shot with MP-E 65mm f/2.8 1-5x Macro Photo or EF 
50mm f/2.5 Compact Macro with Life Size Converter EF, the slider 
indicator changes to [Magnification factor].
The [Shooting distance information] slider is applied to all the 
correction items with check marks.
Shooting Distance Information Slider