Lytro Illum User Manual page 78

The refocusable range varies widely, depending on the zoom and focus positions of the camera when a picture is captured.
The default optical offset, with the focus position used for the live view and for
autofocus set at the peak of the near refocusable range
Depending on the style of photography and the subjects being shot, it may be advantageous to relocate the optical offset
position. As an example, when shooting landscapes, set the optical offset to the peak of the far refocusable range. Then,
when composing a shot by focusing on a distant object, most of the refocusable range would be available for closer objects.
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1. Near peak
2. Default optical offset