An outline is given of the factors which can lead to errors in the experimental measurement of 0.t.f. and therefore to disagreement between the results obtained by different laboratories. Some of the subsidiary checks which can be carried out on 0.t.f. equipment to eliminate or reduce these errors are indicated. Methods of establishing the overall validity and accuracy of such equipment are described and their relative merits discussed. This covers the work done by the 'Sira' group sponsored project on simple standard test lenses and includes a discussion of design requirements, the limitations on the applicability of this method and proposals for extending the work.