Determination of the focus point for measurements of the modulation transfer function MTF

\nAt PTB, a multi camera MTF measurement setup is developed for the traceable determination of the modulation transfer function (MTF) of objectives. In recent works the MTF of a reference objective is determined within the image field in an angular range of up to ±20° to the optical axis. For the investigation of angular dependency of the transfer function the MTF for a given spatial frequency is evaluated at a datum point. A common choice of the datum point is the focus point Pf.\nThere are different methods to obtain the focus point from the measurement data. In this contribution several approaches for the determination of the focus point are investigated and compared. In addition, owing to the temperature depending extension of the measurement setup the position of the focus point strongly depends on the ambient temperature. We present also some results of the investigations on the influence of temperature changes on the position of the focus point.\n