Centre Nodal Point.
I wrote this for myself sometime back when I was considering buying a second quick release plate for my arca swiss style tripod which led me to investigate if doing so could improve the parallax errors in the panorama photo's I sometimes stitch together using Hugin open source software.
The Centre Nodal Point (C.N.D.) is the point on the lens about which it must be rotated in order to eliminate parallax when joining images to make a panorama. It is often around the middle of the lens and varies with the focal length of a zoom lens (although - "FYI the correct term is either the centre of projection, optical centre or "no parallax point", the most widely used term: "nodal point" refers to something else! – Matt Grum" in the questions comments disputes this term).
The tables below were taken from www.olypedia.de. They show the distance from the sensor plane to the C.N.D. and the adjustment required for the distance from the sensor plane (θ) to the tripod mounting screw hole.
Table 1: sensor plane adjustment.
to sensor mount (+1mm) to sensor mount (+1mm)
11mm 113mm 114mm - -
14mm 108mm 109mm 76mm 77mm
18mm 103mm 103mm 74mm 75mm
22mm 98mm 99mm - -
25mm - - 74mm 74mm
35mm - - 70mm 70mm
42mm - - 56mm 56mm
Table 2: distance from C.N.D. to sensor plane and mount.
These figures correspond roughly with those from www.myfourthirds.com by Klaus Schraeder (this link appears to be broken now) for the 11-22mm lens on an E-1 camera when adjusted for the sensor plane distance.
to tripod mount for E-1 mount to sensor plane
11mm 95mm 19mm 114mm
14mm 90mm 19mm 109mm
18mm 85mm 19mm 104mm
22mm 80mm 19mm 99mm
Table 3: distance from C.N.D. to sensor plane and mount for E-1 after Klaus Schraeder
To find the sweet spot for a lens set the camera on a levelled tripod focusing the camera at two vertical targets separated by as large a distance as possible. When panning the camera from side to side you will see the targets move in relation to each other and the background if the lens is not properly mounted. Move the lens forward and back on the mount until the least amount of parallax is evident in the viewfinder.
These images above demonstrate to parallax effect created by panning the lens across a scene with one nearby object (the rock) appearing to move in relation to a distant one (the tree line), and below the effect after the camera's position was corrected.
www.manfrotto.com suggested (I can't find the article anymore) starting with the front lens element over the tripod mount axis and moving the lens forward until the parallax error is eliminated.
I found I could use a flash bracket mounted on top of my existing 50mm quick release plate to gain the necessary offset, though the length of the bracket in front of the lens meant it entered into the frame on wide angle shots. By trial and error I found that the OM 50mm lens on an E-520 sits with the front of the lens roughly over the centre of the tripod head and the 11-22mm sits near the front of the focus ring when at 22mm.
If you want more parallax diagrams try this answer by rafael.