In this proceeding a new approach for the description of geometric-real threads will be given. The holistic determination of the virtual pitch diameter depends on the whole thread scanning. For this purpose, the optical focus variation system, consisting of a microscope objective and a digital camera, captures a stack of pictures and reconstructs the thread surface by a contrast analysis between neighbored pixels. A special calibration gauge for the focus variation system was designed and tested. The traceability and the measurement uncertainty for 3D-Points in precision thread gauge measurements will be given. The uncertainty in the current measurement setup is < 0.6 μm in each coordinate direction (x, y, z) for a S11.2 thread.