To Steve's question: When I say I sometimes have to "undo" movements temporarily to be able to focus through the centre of the image circle, I mean lowering a rise or raising a fall or reversing a shift, in other words, undoing temporarily movements that in cases where such action does not alter the plane of focus. Obviously this won't work with a tilt or swing. The point I was trying to make, perhaps poorly, is that it is not always abvious where one should look to adjust focus, and I have found that the best focus is almost always obtained by magnifying the image at the centre of the image circle no matter where that falls on your fresnel screen after displacements are made.