  The distinction between point and line resolution in transmission electron
microscopy (TEM) arises because an ability to image sub-0.2 nm fringes is a
necessary, but not a sufficient, condition for imaging individual atoms. In
scanned tip microscopy, as in TEM, empirical data on instrument response should
precede assertions about point resolution. In the ``slow scan limit'',
time-domain noise and geometry effects decouple, and tip shape can take on the
role of a 2-dimensional impulse response function. We indicate here that
nuclear track pits can be used to quantitatively measure tip geometry with
nanometer-scale resolution in three dimensions, that stationary tip images
provide a robust measure of time-domain instabilities, and that when these data
are taken before and after imaging an unknown, images with instrument response
quantitatively constrained by experiment are possible. Specimen-induced tip
effects also become measurable in situ.
