If voltage determines the brightness of a LED, how does amps affect it?
You cannot control voltage and current independently. Here's the first image result I got for "LED voltage-current curve": 

If your LED is at 3V when current is 30mA, it'll probably be somewhere around 2.9V at 15mA. 

Now that this is cleared up, LED brightness is roughly proportional (in photon count) to the current, so with 15mA you'd produce about half the brightness than if your LED were at 30mA. It won't look that way though, since our vision is kinda logarithmic, so half the photons doesn't look "half as bright".