Focus Repeatability

["Tom Droege" <droege@snapmail.us>]

Reading back messages I am reminded that I threatened to put a dial
indicator on the camera to check the focus.  I did it.

It repeats exactly! Well the dial indicator has divisions of 0.001" and it
repeated to about 0.0002".  The accuracy was not great.  About 3% low if
my idal indicator is accurate.  i.e. when commanded to move 0.500 it only
moved 0.485.  But it is hard to line up the dial indicator exactly, so the
accuracy may be OK.  I previously found these lead screws to be good to
less than 0.1% but you never know when you might get a bad batch.  These
are rolled Acme threads so who knows who was running the machine on the
day the one I tested was made.  For a device that makes 0.001" steps, this
is plenty good enough for the purppose.

Note that I did not see any evidence of sticking.  Hoever, they berings
were freshly lubricated before I started testing.  As the focus was moved
in and out the dial indicator tracked smoothly with no jerking that might
indicate a stiction problem.

Tom Droege