The adaptor itself seems to work fine. I've used it with lenses like the Helios 44M 58mm and Helios 28mm wide-angle with much success. However after shooting with it for several days, I've noticed maybe a dozen fine metal particles covering my A7ii's sensor. I can't be certain whether the particles came from a lens or the adaptor itself, but it might be a good idea to leave one of these adaptors on each M42 lens you have, or purchase a more expensive one that won't wear.
08/05/2019
Excellent adapter, does exactly as it should. I can now use my selection of M42 lenses on the Sony A7ii the way they should be used.
16/02/2019
09/12/2018
Good machining shame wasn't the one I needed a lot of forums misleading about the adapter I needed but will use soon when have the right lenses
08/12/2018