Cinematography-Theory-And-Practice

Figure 14.1. (previous page)Extremely narrow depth-of-field isessential to this frame from Seven.Figure 14.2. (above) A zoommounted on an HD camera.PHYSICAL BASIS OF OPTICSExcept for certain minor differences, the principles of optics and theuse of lenses are the same for film and video. Nearly all principlesof optics and optical design are based on a few properties of physics.The two most basic are reflection and refraction. There are a fewthings we need to know about the basic behavior of light in order tounderstand the fundamentals of optics.Aside from lighting sources, most things in the real world do notemit visible light but reflect natural or artificial light. The reflectionof light can be roughly categorized into two types of reflection:specular reflection which is defined as light reflected from a smoothsurface at a definite angle, and diffuse reflection, which is producedby rough surfaces that tend to reflect light in all directions. There arefar more occurrences of diffuse reflection than specular reflection inour everyday environment.The basic rule of reflection, known to any schoolchild, is: the angleof incidence equals the angle of reflection. The amount of light reflectedby an object is dependent upon the texture of the surface. When surfaceimperfections are smaller than the wavelength of the incidentlight (as in the case of a mirror), virtually all of the light is reflected.In everyday language — it’s shiny. However, in the real world mostobjects have convoluted surfaces that exhibit a diffuse reflection,with the incident light being reflected in all directions.As will be discussed in the chapter on lighting, diffusion is also akey element in controlling light that is transmitted through things:namely diffusion materials. In both cases, diffusion means the samething: the light rays are more scattered in all directions after theyare reflected or transmitted than they were before. The opposite ofreflection, absorption, is of interest in two ways. First of all it is howobjects have “color” (see chapter on Color Theory) and secondly it ishow we control light on the set.RefractionThe refraction of visible light is an important characteristic of lensesthat allows them to focus a beam of light onto a single point. Refraction,or bending of the light, occurs as light passes from one mediumto another when there is a difference in the index of refractionbetween the two materials.Refractive index is defined as the relative speed at which light movesthrough a material with respect to its speed in a vacuum. When lightpasses from a less dense medium such as air to a more dense mediumcinematography270