The goodman focus value ranges from \(-2000\) to \(+2000\) the best practice (from experience) is to sample every \(200\). If you want to refine you should not go below a \(100\) step. If you already know the approximate focus value for a given setup, you don’t need to do the full range, just take enough samples so it is possible to make a fit or see the trends. At least five points.
Always use the narrowest slit available
For imaging, an image of the slit is obtained, illuminated by an uniform light such as a quartz lamp. A special ROI is used for faster reading. This does not affect the focus of course.