When I look at a 50W constant current LED driver, it has the following specifications:
Input: 85-265VAC
Input: 0.5A
Output: DC 24-36V
Output: DC 1500mA

Ok, ignoring efficiencies, to power a 50W LED with 1.5A, the voltage should be about 33.33V (which is in the output voltage range).
To get 50W out of the driver, you need at least 50W in. Therefore, an 85V input should draw 0.59A and 265V should draw 0.19A.
Questions:
1. Why does the driver just say 0.5A? Is this a max?
2. At 85V, will the LED be under driven?

 

1. Why does the driver just say 0.5A? Is this a max?

Yes, because you want to know what kind of fuse or breaker to use.

2. At 85V, will the LED be under driven?

I don't really know. Probably the current, voltage, watts will be slightly low. Just on the edge of falling out.

Numbers lie. lol
The supply works from 85 to 265vac. Some of the ratings were probably taken at 120Vac and 220Vac not at min or max.

 

1. 0.5Amp is the current to use for power consumption figuring

2. Nobody would power it with 85 volts. That is a totally non standard voltage. That is probably the average voltage for the control to stop functioning.