You might get a kick out of these…
I used to build lots of CNC machines and always loved laser engraving/cutting rulers out of craft sticks to prove their accuracy. I calibrated my machines to create rulers that very closely matched a commercial metal ruler. This was also one of the few videos that I added a sound track… 
One thing this video does show fairly well is the top half of the long “hour-glass” shape of the beam. The longer/taller the hourglass is, the longer the thinner area of “reasonable focus”. A short squatty hour-glass shape supposedly gives a sharper focus but lacks the range of “reasonable focus” that the longer has.
I have a dozen or so laser modules but usually only have a couple in use at any given time. All can be checked and adjusted this way. I like to use 50mm as the focus distance… insures a nice long “hourglass” shape and good range of “useful focus”. Also gives best performance with cutting operation… straighter sides, deeper cuts.