If I have a led hooked up to a 12v DC supply with a single resistor it runs nicely as you would expect. If I add a second LED in parallel the brightness drops, slightly, the brightness on both is (to my eye at least equal). My antiquated primary school knowledge tells me in parallel they should draw more current and stay the same brightness.
What's more if I add any number of additional LEDs the brightness doesn't drop again, its only after the second one.
I've found some questions and answers that indicate that running multiple LEDs with a single resistor isn't a good idea because the current will be different on each LED and that makes sense. However I'm keen to understand what is actually happening to cause the LEDs to dim.
What is wrong with my understanding of bulbs in series vs in parallel and why does a second bulb cause the brightness to drop and additional bulbs not?