Using a laser with a beamsplitter and a lenseless video camera, the optical axis and true focus of a 16mm camera and lens is determined. The laser is spatially filtered and aimed through the beamsplitter into the camera lens. The return reflection from the film through the lens is observed on a video camera looking into the beamsplitter. The observed return beam converges when the lens is focused. The maximum intensity spot seen by the video camera is the mirror image of the pinhole in the laser spatial filter. Calibration targets can then be placed at the true object plane. The slightly defocused interference pattern can be used to determine the alignment and centering of the lens/camera system. The result is shown to be an improvement in the quality of the analysis results from the camera used for measurement of high speed motion.
David G. Labick,
Judson B. Estes,
"Photometric Camera Alignment And Focus Using Reflection", Proc. SPIE 0954, Optical Testing and Metrology II, (16 January 1989); doi: 10.1117/12.947651; https://doi.org/10.1117/12.947651