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

At PTB, a multi camera MTF measurement setup is developed for the traceable determination of the modulation transfer function (MTF) of objectives [1,2]. 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 P_f. There are different methods to obtain the focus point from the measurement data which show different uncertainty. In this contribution several approaches for the determination of the focus point are investigated and compared. [1] Schenker M., et al., “Effects of misalignments on the modulation transfer function measurement of camera lenses analyzed in optomechanical simulations,” Opt. Eng. 59(3), 1 -17 (2020). [2] Schake M., Schulz M., “Influence of camera temperature on MTF measurements with finite image distance,” European Physical Journal Web of Conferences, Vol. 266, p 10018 (2022).