Quite sometime ago I purchased a 610mm AeroTessar, it's real heavy, an extra tripod/light stand has to be used to support under the ext. bed of my Toyo 810 to support this behemoth, and everything has to be dis-assembled to reframe.  It's just an ordeal to use.

On the plus side which excited me in buying this lens in the first place was of course 610mm worth of lens which w/a fast wideopen aperture of F6.  I also planned to experiment w/this lens by unscrewing the rear element of the front cell to induce spherical aberration similar to the way it's done w/my Velostigmat II w/its diffusion ring.  

The rear element has a metal casing w/threads, and it screws into the casing which contains the front front cell, I couldn't unscrew the rear element out of the front cell, it was jammed.   After several attempts to unjam the rear glass,  I gave the lens to Dean @ Dean's Camera Repair and he went to work.   Nothing worked until he bored out a small hole in the casing of the rear element, and inserted a small tool into the hole and taped on it approximately 1000 times according to Dean's reckoning, over the course of 3 1/2 weeks before he worked the rear element loose.

Out of curiosity I asked Dean why he thought the very light taps would work and he said that each tap set up a vibration and that sooner or later w/repetitive tapping that the element would work loose.  He was right.

This is the image of the rear glass leaning up on where it screws into the front cell, the 253.5 AeroTessar I bought later is constructed in exactly the same way as the 610mm you see here.  There's something about either the glass or how it's been polished, and there's a sparkle to it, particularly on my 253.mm which is in better shape than the 610mm which looks better than almost all of the glass I have, even better than my 250mm Apo-Tele Xenar which I bought new for my Silouette 612 camera.  The glass is simply beautiful, it looks as if you're looking into a perfectly still body of water and not glass, and this is an old lens, I wonder why the glass on this lens stand apart as it does w/it's clarity and sparkle.

The 610mm is bigger in scale than either my 300mm Velo II or the 253.5 AeroTessar, thus the rear glass can be screwed out and away from the the front cell approx. 1/16" farther than the 3/8" maximum of my Velostigmat II diffusion ring w/its set screw removed.  

I got to solve the logistics involved in shooting w/the giant, but I'm going to do despite the ease of using the 253.5mm lens because of its FL and still fairly wide aperture which I'm hope will render some special SM in a spectacular way