Originally posted by Serkevan About 40mm from what I had measured last time I did that. For reference, the Nikon 20mm f/1.8G has a front element measuring about 47mm or whereabouts.
Originally posted by house 45-50mm (I measured ~47mm but accuracy is not good due to the images I have available)
Both estimates indicate that the maximum aperture could be at least F2.8, but possibly F2. The old FA20, of which I have a copy that is quite good IMO, is F2.8 with a front element of 43mm diameter. Of course, the entrance pupil is used to determine the maximum F-number, not the size of the front element, and the DFA21 is in all likelihood a quite different design to the FA20.
I’m interested in how this lens turns out. Faster would be better with an optical viewfinder, but DoF becomes of less interest in this focal length, except perhaps for closer work.