There are optical shops on every corner in Hong Kong. Walk into any of them and you'll be given a free optical test by a guy in a white coat. Whether these people are qualified optometrists, I'm not sure but I've never had any problems. The main test is performed by machine, with a few other quick tests for astigamtism (?). It usually takes a couple days to have your glasses made.
The last time I went to the optical shop inside Sogo in Causeway Bay (I think it's called Tokyo Optical). A middle-aged Japanese gentleman performed the tests and seemed more thorough and professional than the staff at chains like Optical 88 and The Optical Shop. Please avoid New World Vision. I once purchased a pair of sunglasses from their Times Square store. I brought them back when the frame became loose. The staff, unaware that I understand Cantonese, admitted that there was a problem with the hinge. After conferring in my presence, the supervisor advised the other man to just tighten the screw and see if I would return.