I may be open to correction but I understand that range finders like the Bushnell work by measuring to a laser device on top of the flags thereby giving a direct reading of the distance to the flag.
However as has been discussed on here before one of the problems with this type of system particularly for erratic players like me is that you need to be able to see the flag to get the reading. That is why I opted for a skycaddie which maps the course and uses GPS technology to give you readings to various reference points (bunkers etc) and to the front middle and back of each green. That way when I've blasted it into the next postcode at least I still have a yardage to the green or at least back onto the right fairway