how to modify a dimmer switch (230V,50Hz supplying 5 incandescent bulbs of 40W each) to get max. brightness variation?

there's a dimmer switch connected to 5 lamps. Problem is the dimmest light occurs halfway of the span of the dimming knob. How do i tinker with the dimmer so the dimmest light occurs nearer the OFF position?

If dimmest is in the middle, then the half the voltage across the pot is needed for your minimum dim.

You'll have to work out which end of the pot sets maximum, it might be zero volts, or it might be the top end.

If you know the pot resistance, then if you added a resistor of equal value at the right end, then as you swing the pot from one end to the other, you'll get the full change you need, over the full travel of the pot.