jsg But High Point Scientific, which sells the adapter, states that it is 4mm in thickness. Which is it?
The physical thickness is of course non-zero.
The way you use the adapter is to thread the M42 (female) part over a 42mm threaded male part. The outer thread of the adapter converts said part from male 42mm thread into a 48mm male thread, but adding zero optical path.
I.e., the part that you linked to coverts a M42 male thread into an M48 male thread, with zero optical path increase. Make sure this is what you need before buying it.
By symmetry, it can also be used to convert a M48 female thread into a M42 female thread, also with zero additional optical path.
Chen
