This is an adjustable 42 mm extension tube that adds 8-12 mm of length. Thread is the standard 1.8" filter thread size of M42 X .75. The depth of the male thread is 8.3 mm and there is a thin (1.72 mm) threaded locking ring on this thread to adjust the spacing. The female thread is 5 mm deep. The body is 1.8" in outside diameter.