We needed to focus the system now and then – but it turned out that this was when the wrong filter had been selected. In view of this, there may never have been a focus problem – as long as the filter was correctly selected the system would proceed to set focus to that filter.

There was never an automatic ‘set the focus’ operation, but perhaps this was not needed? It would have been required if we had gotten the NDs up and running as a known laboratory-tested focus point for these may not have been at hand, and then it would have been necessary to find it by hand and setting a fixed value.