This is a flashbulb from my venerable Armato strobes sitting on my plano-convex magnifier used as a base.  In order to get the bulb to stay put on the flat side of this magnifier, I used a piece of double stick tape on the the bottom of the bulb and secured the bulb to the magnifier.

The diffusion control on my Velstigmat II was screwed out to a 'default' position about 2/3 of the way away from the barrel before the front ring/front element comes all the way off.  At this default position I feel I get a nice compromise between 'crisp'/smooth transitioning to out of focus.

This took quite a bit of focusing back and forth to get a nice compromise focus between the filament inside the bulb and the reflected shadow of the bulb on the magnifier and the light pattern on the front edge of the magnifier.