158
Figure 9.14 The reproduction of the coloured pencils shown below when photographed on (centre) ortho, and (far right) panchromatic monochrome films
Films, filters
tone than we would judge them to be. The difference is generally accepted, and is of some value in allowing slow pan materials to be handled under very deep green darkroom safelighting. For a more exact match you can shoot using a yellow camera filter. A few black and white films are made insensitive to the red end of the spectrum beyond about 590 nm, and are known as orthochromatic (‘ortho’). These films reproduce red as black on the final print, and orange as very dark. Ortho materials – mostly sheet film – are useful for photographing black and white photographs or drawings not involving colours. You can conveniently handle them in the darkroom under red safelight illumination. They are also used for some forms of medical, forensic and scientific photography. Ortho film speed rating is lower when the subject is lit by tungsten light rather than daylight or flash, because the former contains a higher proportion of red wavelengths. One or two films, very slow and intended for the printing darkroom rather than camera purposes, are made with blue (and UV) sensitivity only. You can also buy special films for camera use that are insensitive to almost the whole visual spectrum but respond to infra-red and UV; see Advanced Photography. Colour films, both negative and slide types, often have a stack of six or more emulsion layers. They use three different kinds of colour sensitization. The top emulsion is sensitive to blue only; others respond to blue and green; and the remainder primarily to red. A yellow filter may be incorporated below the blue-sensitive layer so that blue light cannot proceed farther into the film. Therefore you effectively have a multi-layered emulsion responding to blue only, green only, and red only – the three thirds of the spectrum. (Later each of these three emulsion types will form its image in a different coloured dye, to reproduce the final image in full colour; see page 313.) The relative sensitivity of the different layers (colour balance of the film) is carefully controlled during manufacture. Most colour films are balanced to give accurate colour reproduction when the subject is illuminated by daylight or flash (5000–6000K). You can also buy a limited range of tungsten-balanced films which have slightly slower red-sensitive layers to give correct reproduction of subjects lit by redrich 3200K tungsten lamps. Apart from ‘daylight’ and ‘tungsten’ colour balance films, one or two exceptional materials are sensitized to infrared or laboratory light sources.