I've just acquired the newer design El Nikkor 150 (newer, that is without a knobbly aperture ring, and with a 50mm and 39mm threads). I'm going to compare it to my Rodagon 150, and perhaps 135, all for use with 4x5 black and white.

I was wondering if there is a disadvantage/advantage to mounting this newer El Nikkor using its 50mm thread, without the 39mm adapter. When the 50mm thread is used, the rear element is more exposed, and when using the 39mm adapter, the rear element is more concealed, visible only through the adapter and what looks like its flare-reducing barrel. I can mount both ways, I wonder if anyone has compared the two approaches with this lens?