Canon EOS 5D Mark III Instruction Manual page 252

u Magnified View
3 Magnification Settings
1x (no magnification)

The image will not be magnified. The magnified view will start with
the single-image display.
2x, 4x, 8x, 10x (magnify from center)

The magnified view will start at the image center at the selected
magnification.
Actual size (from selected point)

The recorded image's pixels will be displayed at approx. 100%. The
magnified view will start at the AF point that achieved focus. If the
photo was taken with manual focus, the magnified view will start at
the image center.
Same as last magnification (from center)

The magnification will be the same as the last time you exited the
magnified view with the <x> or <u> button. The magnified view
starts from the image center.
With images taken with [Live mode] or [u Live mode] (p.207), the
magnified view starts from the image center.
252
Under the [33] tab, when you select
[Magnificatn (apx)], you can set the
starting magnification and initial position
for the magnified view.