What is the Difference between brightness 400cd and 500cd?

Answer 1

100cd. . . ^_^

I guess it depends on what you are doing with the leds. If you are trying to illuminate, think of it like a camera f stop. One stop brighter is a doubling of brightness. So the 500 CD is only a quarter of a stop brighter than the 400cd. Not much brighter, so if it is battery operated you might defer to longer battery life and stay with the 400cd. 600cd though would be a worthwhile step up.

If it is an indicator, the apparent difference, depending on the color and room ambience, could be significant, so go for the brighter led.