The Microprism Focusing Screen for Leica S2 Camera from Leica is an accessory interchangeable focusing screen that greatly facilitates manual focus using the Leica S2 camera. This would be particularly helpful, for instance, in using the 120 mm macro lens, where manual focus is preferred over autofocus for more control and precision. The Microprism Screen has a more granular visual characteristic which make the focus "pop" more quickly and obviously than a standard focusing screen.
After all these years, this is hard to find. Ace Photo has a few for sale. I look forward to finding otherb items that may be hidden on their shelves.
