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I just installed new recessed lighting in my finished basement, wiring 4 fixtures with dimmable led bulbs to a dimmer switch. Additionally, 2 of the fixtures are on their own standard switch after the dimmer so that I can turn off half the lights on the dimmer circuit when desired.

Everything works as intended but I have noticed a strange phenomenon. Starting with the secondary switch off so only two lights are on, and the dimmer switch is on a low setting, if I turn on the second set of lights, the lights that were on originally get noticeably brighter when turning on the second set, and then all 4 are at the same brightness.

Does the effectiveness of a dimmer switch reduce when more lights are added in parallel to the dimmer circuit? The answer seems to be yes, but why?