Dimmable ballast question

If I have a 400w dimmable/switchable ballast, when i dimm it to 50% do i need a different bulb or will the 400w just operate at a lower output?

Same bulb.

Hi zips, you use the 400 watt bulb. Although bulbs are designed to operate at a certain voltage. so, dimming the ballast will change voltage and burn the elements in the arch tube at a different temperature. doing this will alter the spectrum your bulb puts out. saying this i would only dim if absoltuely necessary.