Cinematography-Theory-And-Practice

Red SensorBlue SensorDichroic filterGreen SensorFigure 9.3. (top) The prism block ofa 3-chip camera.Figure 9.4. (above) A Bayer filtersensor, which is used in single-chipvideo design. Note that there aretwice as many green pixels as theirare blue or red pixels.TYPES OF VIDEO SENSORSDigital video requires something that serves the same function asfilm, which is, after all, just a medium that records and reproducesthe patterns of light and dark that are focused on it by the lens. Indigital video, electronic sensors perform this function.Two types of video sensors are widely used: CCD (Charge-CoupledDevice) and CMOS (Complementary Metal Oxide Semiconductor).Each type has its advantages and disadvantages. A CCD consists ofa photo-voltaic region containing capacitors which convert lightenergy to an electrical charge. As with the silver based grains inmotion picture film emulsion, a minimum number of photons arenecessary to cause a change. Tiny lenses focus the light onto eachof the millions of capacitors on the light sensing layer. CCDs aremost often referred to my how many pixel elements (the capacitors)they have — measured in megapixels, or millions of pixels. This hasbecome a shorthand for the quality of an image sensor, but there areother factors that determine image quality as well. CMOS sensorshave the advantage of high noise immunity and relatively low powerconsumption.Three-Chip vs Bayer Filter SensorsThere are two methods used to record the images: three-chip andBayer filters. Increasingly, high-end pro digital HD cameras often usea single sensor (similar to digital still-photo cameras), with dimensionsabout the same size as a 35mm film frame or even larger. Animage can be projected onto a single large sensor exactly the sameway it can be projected onto a film frame, so cameras with this designcan be made with PL (Arriflex type mount), PV (Panavision typemount), and similar systems of attaching lenses. This means that alarge range of extremely high-quality motion picture lenses can nowbe used on these HD cameras.In a single-chip design, clearly there has to be a way to record thethree channels of color (red, green, blue). One way to do this is witha Bayer filter, which is an array, or mosaic, of color filters that sits ontop of the video sensor. The pattern is G, R, G, B, which means thathalf of the filters are green, which is the dominant color in humanperception. Using this method requires a process of deBayering anddemosaicing before it begins to look like a useable image.Since depth-of-field is largely dependent on the size of the frame atthe image plane, the larger sensors also let these cameras achieve thesame shallow depth of field as 35mm motion picture film cameras,which is important because many cinematographers consider selectivefocus an essential visual tool.Three-Chip DesignThree-chip cameras typically use three 1/3-inch, 1/2-inch or 2/3-inch sensors in conjunction with a prism block that separates theincoming image into its primary colors, with a separate sensor capturingeach color. Three-chip designs have advantages in terms ofcolor reproduction, but they have optical design problems that makeit difficult to use traditional prime lenses (although some speciallydesigned prime lenses have been built for these cameras) and areincapable of achieving 35mm depth-of-field unless used with opticaladaptors that permit the use of lenses designed for 35mm; however,these adapters result in a loss of image sharpness and a substantialloss of light.cinematography150