Fotodiox is normally a top-quality producer, so it may be that I have a bad unit but tested it too late to make a return. In any case, the product causes any M42 lens that I mount to be unable to focus...the ring gets stiff at either end whereas when mounted to a true M42 mount camera (such as my Mamiya DSX 1000) there are no issues. I had this problem with every M42 mount lens I own, meaning that this adapter is useless. The same stiffness is present when setting the aperture.
I have had no issues with any other Fotodiox product before this particular adapter, so do not take this product review to be a negative assessment of all Fotodiox products, which are typically exceptional in quality.