40W ballast., 20W bulbs?

[XeniaMania]

I just want to verify if this is correct. If I have a flourescent ballast that can drive 2x40W bulbs, but I use 2x20W actinic bulbs instead, I'd overdrive the bulbs reducing their life, but the ballast should be ok right?

 

[lastin1]

If the ballast is magnetic, it will burn out the bulb very quickly. If it is electronic, it will probably adjust to the lamp.