The forward voltage drop is a nearly constant value for any given LED, and it varies with LED models. But, the current is widely variable, and it is by varying the current alone that we adjust the amount of light emitted by the LED. One advantage of Cree LEDs is the great data packages that are provided for all of their LEDs. The cheap ones we can buy on Ebay, for example, don't come with that data. And, LED light fixtures don't come with any data at all, except, for some, the PAR vs distance.