I use the panorama app that comes with touch controller and focal lenght values from 450 and above all give the same message and can't be added to the list
Ok - Josef will be back from vacation this weekend. Afaik it´s the Merlin´s lack on mechanical precision above 400mm - that´s understandable at the price: it can´t position such small angles precisely enough. But i´ll ask Josef nevertheless to post an answer here.
If you want something you´ve never had, then you´ve got to do something you´ve never done.