A compensating arm is provided extending from the proximal end at a joint to the distal end of a jib arm and arranged identical in the horizontal length to the jib arm. The compensating arm is pivotally joined at its distal end to the distal end of the jib arm and at its proximal end fixedly joined to a pivot which extends across the joint of the jib arm, whereby the pivotal movement of the jib arm about the joint is measured by an encoder which incorporates a compensator and is mounted to the joint with compensation of the relative movement of the pivot to the joint. This allows the encoder to detect the deflecting movement of the jib arm with the relative movement of the pivot to the joint compensated and thus locate the focal point correctly in the vertical direction.