Fall-off
Light intensity tends to drop off over distance. For a point light source, intensity decreases as distance increases. Intensity is proportional to distance−2. For a thin infinitely long light source, intensity is inversely proportional to distance. For a light source of infinite area, intensity does not decrease at all. Generally, a soft light source does not drop in intensity as quickly as a point light source would (as distance increases).
Certain lensed lighting instruments (i.e. Ellipsoidal reflector spotlights) have a good deal of "throw" and do not lose much intensity as distance increases, much like soft light sources. These light sources tend to be more effective at large distances than soft light sources. At large distances, an effective soft light source would have to be very large! The (mostly) parallel rays of such instruments tends to cast hard shadows, unlike soft light sources.
Read more about this topic: Soft Light