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How changing power supply voltage affect the LED? Answered

There is one example:
- With a LED with a 3V voltage drop, a 9V source, and 20mA operating current then 300 ohms is the right resistance
- But it's best to increase it to 330 ohms, at a cost of brightness

1.- Then what happens if source is dropped to 6V and resistance to 150 omhs? Will it be dimmer than with 9V source? Same brightness?
2.- Is there a way to calculate brightness in general, with given a setup?