quote out manual:
Focal length and focusing distance
The lens focal length and focusing distance of a photo are recorded in the EXIF data of your images. However, this information is not always accurate. For example, different but close positions of the focal length ring (say, 17 and 18 mm) could result in the same value (say 18 mm) being recorded in the EXIF data. In this case, the distortion correction may be less than optimal. In the same manner, the focusing distance might be recorded in the EXIF data with insufficient precision, and similarly lead to an imprecise correction. In both cases, to improve the effectiveness of the optical corrections, you can provide more accurate values in one (or both) of the rollups that appear in the Geometry palette:
Focal length : Use the slider to specify the lens focal length.
Focusing distance : Select a range for the focusing distance in the drop-down menu, then fine-tune with the slider.
The Focal Distance and Distance focusing sliders are permanently displayed in the OS X version, and appear automatically in the Microsoft Windows version.
Still don’t understand what it does exactly. The help manual isn’t elaborating so to speak.
That is stil a major thing: to slim explanation how things work and how you can use it along other tools. (We posted earlier about which tools interfear/effect each others automated settings and what they actual are using to accomplish its out come.)