Apple Shake 4 - Manual (Page 215)

Chapter 6 Importing Video and Anamorphic Film 215
The blur now looks proportionately correct.
Rendering Squeezed Images
Once your composite is complete, reset the proxyRatio (in the Globals tab) to 1, and
render. Do not change any other parameters. The resulting image appears squeezed on
the X axis, but this distortion is corrected during the film projection process in the
theater.
Although you worked on the image in a squeezed state, all elements are properly
positioned when the proxyRatio is returned to 1. Use of the proxy system temporarily
squeezes the image down and then stretches it back out, thereby maintaining the
pixel data.
Handling Video Elements
Nearly all standard-definition video formats use nonsquare pixels, which results in a
squeezed image similar to that caused by anamorphic distortion. In addition, each
video format has its own aspect ratio. Using the proxyRatio parameter is not
recommended for video elements because proxyRatio squeezes the image along the Y
axis, which causes problems with field separation.