We essentially count photons- If I remember correctly 160 photons per second per square centimeter equates to a magnitude of about 9.8... far fainter than the human eye can see unaided...

The laser guide star we use has a magnitude that ranges between 9 to 11 (There are rare times when it is brighter or dimmer than this range, but they are rare)

Personally, I would like to see a few observatories do a survey of various stars of widely differing 'brightness' and compare the measured magnitudes- I am willing to bet that they would all get slightly different results for the magnitudes... I don't think it really matters, but it would be nice to have an absolute calibration between observatories...