What is the theory behind low voltage focus CRTs?

[ARC Tech-109]

I notice the same effect on my Panasonic Tau series with the velocity modulation, basically the same effect using a magnetic field to squeeze the beam. It has a horizontal & vertical plane focus and when they're adjusted right I can clearly see the moire from the mask. The problem with this is running in 1080i I get a third beat on the fine details and patterns and have to run the focus out slightly, granted this is a slot matrix but the effects are about the same and it seems like a step back given the limitations of the CRT. This is one reason why I'm not going LCD any time soon, too much emphasis on the fine details at the cost of the big picture. Just my $0.02, half went to taxes.

I do have a 1970 model Astro Sonic with all the bias and drive controls plus the tube focus rectifier that would benefit from this. At some point I will design a betacam component input interface for it using tubes, that will give it something to show.