Hard and soft light are different types of lighting that are commonly used in photography and filmmaking. Soft light is light that tends to "wrap" around objects, casting diffuse shadows with soft edges. Soft light comes from a light source that is large relative to the subject; hard light from one that is small relative to the subject.
The hardness or softness of light depends mostly on the following two factors:
The softness of a light source can also be determined by the angle between the illuminated object and the 'length' of the light source (the longest dimension that is perpendicular to the object being lit). The larger this angle is, the softer the light source.
Soft light use is popular in cinematography and film for a number of different reasons:
Hard light sources cast shadows whose appearance of the shadow depends on the lighting instrument. For example, fresnel lights can be focused such that their shadows can be "cut" with crisp shadows. That is, the shadows produced will have 'harder' edges with less transition between illumination and shadow. The focused light will produce harder-edged shadows. Focusing a fresnel makes the rays of emitted light more parallel. The parallelism of these rays determines the quality of the shadows. For shadows with no transitional edge/gradient, a point light source is required. Hard light casts strong, well-defined shadows.
When hitting a textured surface at an angle, hard light will accentuate the textures and details in an object. This will also increase the 3D-appearance of the object.
Light intensity tends to dim with distance. For a point source of light, intensity decreases as distance increases. Intensity (I) is inversely proportional to the square of the distance (D), as expressed in the formula I = 1⁄D2.
For a thin infinitely long light source, the intensity is inversely proportional to distance. For a light source of infinite area, the 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 (e.g. ellipsoidal reflector spotlights) have a good deal of "throw" and do not lose much intensity as distance increases. 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 tend to cast hard shadows, unlike soft light sources.
Most light sources have a non-negligible size and therefore exhibit the properties of soft light to some degree. Even direct sunlight does not cast perfectly hard shadows.
In "hard" light sources, the parallelism of the rays is an important factor in determining shadow behavior.
The quality of light can be altered by using diffusion gel or aiming a lighting instrument at diffusing material such as silk. When shooting outdoors, the cloud cover provides nature's version of a softbox.