Influence of optical bench errors on accuracy of otf measurements

Comparisons of the results of otf measurements made in different laboratories have shown that wide differences often occur at moderate to large field angles. A major cause of such differences may well be mechanical error in the optical bench which supports the lens under test. Ideally, the bench is required to be of such accuracy that the image-analysing system moves in a plane (the chosen image plane) parallel to the lens mounting face. Mechanical errors which cause the analyser to be displaced from this plane by only a few micrometres, may cause significant variations in the measured otf. In this note a method is described for measuring the perpendicular displacement, δz, of the image analyser from the image plane at different angular settings of the optical bench.