Earlier in this blog I mentioned having SMCP FA 100/3.5 Macro lens which goes only to half lifesize as it is. This lens is said to be designed by Cosina and it is sold under a number of different brand names. Vivitar, Cosina, Phoenix are some of them. All but Pentax include an additional piece - the "matched macro adaptor (1:1)" (the spelling is original, though word 'adaptor' is marked by red underline on my screen, supposedly indicating that it is misspelled). With the help of my friends I recently got this adapter (I'd rather not upset my spell checker ;-) ). I should say that I am impressed. It is very solid two element (?) multi-coated filter that can be used on any lens with 49 mm filter thread In fact, I tried it on DA 21, FA 43 and FA 77 lenses and found it perfectly useful. Optically it seems to have no visible negative impact on image quality, including corner sharpness, though I admit I did not inspect my images in pixel peeping mode. It seems to bring some chromatic aberrations (if shooting a printed or written text from an angle), but its merits far outweigh any possible disadvantages.
Highly recommended especially if you have several lenses with 49 mm filter thread.