But for many lamps, mainly the saturated vapor types, you should implement multiple steps if you want to dim deeper (2 steps or so for 50%). Reducing power by more than about 20% at a step brings you a risk of the lamp extinguishing, as the electrodes tend to cool down faster, so get higher cathode drop faster than the arctube cooling brings the pressure so the anode column voltage down to make the thing stable.
The issue may not directly appear with a fresh lamp, but once it wears a little (mainly the blackening making the arctube to run hotter), it starts making troubles.

@sol:

Dimming a MH lamp is generally not a good idea when you care about light quality. [...]

I can see that this would be a problem that I didn't think about. Makes sense. This would be to modify an existing setup that operates with a Sylvania PowerBall lamp and I am not really interested in getting a special lamp. I might still experiment with it, though.

Thanks to everyone who provided valuable information on this topic !

@ WWHID:
We use this website to compress images, there is a section in the guide on how to do it as well.