Optomechanical analysis of the alignment tolerance of threaded lens mounts

Abstract

Threaded mounts are one of the most common interfaces between optical systems and commercial-off-the-shelf cameras. Popular examples include the established C-mount, as well as the newer TFL-mount which accommodates for larger sensor formats such as the APS-C detector. In all cases, the thread is used to adjust for focus by clocking the optical system with respect to a fixed camera assembly or vice versa. For this reason, the alignment between the datum axis of the optical system and the array detector plane inside the camera depends on both the allowances and tolerances of the thread interface, and on the manufacturing tolerances of the mount components. To highlight how the stack up of these tolerances can affect image quality of an optical system, we first perform an inverse sensitivity analysis to determine the detector alignment specification as a function of system F/#, field of view, and chief ray angle. We then calculate the misalignment contributions of the thread between the optical system and the lock ring that sets the camera axial position for best focus. This optomechanical analysis allows us to determine if thread mounts are appropriate for the specifications of the optical system under consideration and to specify the tolerances of the thread interface when this is the case.