That is very strange - do you know the intended working distance of the objective, and the thickness of your coverglass? 160/0.17 should meant that at its designed tube length,you can easily focus through 0.17mm of glass, so as @ImAreO says there ought to be 0.25mm from the objective to the focal plane (leaving around 100um from the outside of the coverglass to the objective). One thing you can check is whether the 50mm focal length lens is correct - if you remove the objective and point it at a brightly-illuminated object some metres away (e.g. point it out the window), it should not form a sharp image. If you then unscrew the two small screws holding on the camera module, and move the camera module back by around 12mm, you should get a sharp (if very zoomed in and thus quite wobbly) image. If that is the case, then the optics module seems to be working correctly and I am very confused! If that’s not the case, perhaps it is possible that your doublet lens does not have the same focal length as the one I’ve used. Is it a ThorLabs AC127-050-A, or a generic one sourced elsewhere?