A positioning device for a reference surface within a lens port having a longitudinal axis, where a reference surface element able to be extended substantially transversely across the port and withdrawn from the extended position are provided. A transverse movement element to effect such extension and withdrawal of the reference surface are provided and the transverse movement element is operable from a position external of the lens port. The device further includes a camera or camera housing with a reference element of this type associated therewith. The invention finds particular use in the field of underwater photography.