The National Physical Laboratory has sponsored fully toleranced optical designs of five standard reference lenses for the assessment of equipment used in measurement of the Optical Transfer Function. This paper will briefly describe those designs, indicating the particular imaging characteristics that each lens simulates and the tolerances associated with each design if the theoretical performance level is to be achieved. The influence of those tolerances on the mechanical design of the lens mounts will be indicated and the difficulties in manufacture arising from them discussed. Examples of three lens designs have been completed to date. The performance of two designs, measured both interferometrically and directly on an OTF measuring instrument, will be compared with the theoretically predicted performance. The performance of the third design, a triplet lens, has only been assessed interfero-metrically. A comparison of these initial measurements with theoretical performance will also be presented. Progress in the construction of the other two lens designs will be reported.