One of the corresponding distances of object point and image point from the nodal points of a lens. The conjugate distances L _o and L _i and the focal length f of the lens are related by the equation 1/f = 1/L _o + 1/L _i. The total distance from object point to image point equals the sum of the two conjugate distances plus or minus (depending on the design of the lens) the distance between nodal points.