The combination of shutter speed, aperture and film speed/ISO (and light modifiers) are expressed as EV or "Exposure Value".
This number depends on the amount of light present in a scene, either natural light (sun/moon) or artificial (strobe/lamp).
A difference of 1 in EV corresponds to 1 "stop" (+1 stop = 2 x more light). This calculation is valid for both analog and digital photography
A ND (neutral density) filter is a piece of glass you can mount on a lens that attenuates the light without influencing the colors