Filters for Control of Natural Daylight
A family of materials, mostly on polyester plastic-based film, are made for light control in these situations. These are normally supplied in rolls that are from 48 to 58 inches wide (122 to 147 cm). In addition, the 85, and ND3, ND6 and ND9 types are also available as rigid acrylic panels, usually 4 by 8 feet in size (1.22 x 2.44 meters).
Reference should be made to pages 323 and 278 in reading this section. Page 323 lists the MIRED shift values for the various materials, and their effect on sources of two different color temperatures. Page 278 summarizes the filter requirements for each element of the lighting system and camera for interior cinematography against daylighted windows.
When properly applied, sharp focus can be carried through windows treated with either the plastic film materials or the acrylic panels. The panels are particularly useful where wind or strong air movement may cause the plastic film to move and produce visible highlights.
These materials are intended for application at openings (doors, windows, etc.) where natural daylight is entering an interior which is to be photographed at a 3200K balance. The "full" conversion materials available are known as "CTO" and "85." In USA lighting practice, the "85" has been the type most widely applied (it is really a Wratten 85B equivalent). The European practice has been to use the deeper correction such as the "CTO." The choice of filter will obviously be determined by the actual daylight conditions being dealt with, or by artistic considerations.
Filters which accomplish less than the full correction to 3200K are also available, and are widely used to deal with the variations in daylight conditions that may be encountered. They are also used where the artistic effect wanted is different from "natural" daylight (page 367).
Where it is desired to use a daylight balance inside the space in which photography is taking place, the only filter normally indicated for the windows will be neutral density. These are usually required due to the overpowering levels of sunlight which are often encountered in natural settings. Typically these filters are available as either plas tic films or as rigid acrylic sheets. Normally they can be obtained in densities which reduce the incident light by Vi, 1,2, or 3 stops (ND.15, ND.3, ND.6, and ND.9).
Combinations of 85 and neutral density or CTO with neutral density are also available. These are utilized to reduce the number of materials which must be installed in order to accomplish both the conversion and the reduction of lighting level.
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