Series editors' preface - Wood Tools

402 Conservation of Furniture
tive emulsions and most have cyan, magenta
and yellow dye forming couplers similar to
those used in colour negative film.
The key components of the photographic
system are the film, the light source and the
camera. Each of these is briefly explored
below.
The film
Films vary in their sensitivity to light, both in
the quality of the light in which they have been
designed to be used and in their sensitivity –
the amount of light required to form a usable
image. The majority of black and white films
are panchromatic, that is they respond, in
monochrome, to virtually the whole of the
visual spectrum. Their sensitivity also continues
some way beyond blue into the ultraviolet.
There are, however, some differences in
response to light as compared to the human
eye. Orange-red, blue and violet are reproduced
somewhat lighter in tone and greens
darker in tone than we would judge them to
be. A yellow filter on the camera can be used
to give a closer match if desired. A few black
and white films are made insensitive to the red
end of the spectrum beyond about 590 nm.
These orthochromatic films are useful for photographing
black and white photographs or
drawings not involving colour. In addition to
these types, films are also made for special
applications that are insensitive to virtually the
whole of the visual spectrum but respond to
infra red and ultra violet.
The balance of sensitivity of the different layers
in colour films is adjusted during manufacture
to render the film suitable for use in
particular lighting conditions. Most colour films
are balanced to give accurate colour reproduction
when the subject is exposed to daylight or
flash. This corresponds to light of colour temperature
5000–6000 K. Film which is balanced
to give correct colour rendition under tungsten
lighting (3200 K) is also available.
All types of films vary in the amount of light
required to give a satisfactory image under
standard conditions of lighting, exposure time
and processing. The sensitivity of film to light
is denoted by an emulsion speed rating. The
most commonly used system for rating film
speed is the ISO (International Standards
Organization) system. Low numbers correspond
to slow films, in other words films
requiring more light to give a satisfactory
image. The ISO rating consists of two figures,
the first number corresponding to the ASA
(American Standards Association) rating doubles
with each doubling of light sensitivity, the
second number (marked with a degree sign
and corresponding to the European DIN standard)
increases by 3 with each doubling of sensitivity.
Slow films are typically rated ISO 25/15
to 64/19, medium films 100/21 to 400/27, fast
films 640/29 to 1000/31 and ultra-fast films
1600/33 to 3200/36. There is a trade off
between speed, graininess and sharpness of
the image. Graininess is the pattern of clumps
of silver grains that can be seen in the
processed image. If this is coarse it will give a
mealy or powdery appearance to the image,
especially when enlarged. Sharpness of the
image is concerned with the degree of fine
image detail the film can record. It is difficult
to increase film speed without increasing the
size of the silver halide particles and therefore
coarsening the grain. If the emulsion is made
thicker to increase speed by increasing the
number of silver halide particles then sharpness
will decrease as a result of light scattering
within the emulsion layer. Consequently,
slower films are generally preferred where the
sharpest images are required and faster films
are preferred for use in low light conditions.
All film compresses tones. That is, it will not
see the same amount of detail in both highlights
and shadows. If importance of detail is
in highlights then exposure should be reduced.
If important detail is in shadow more exposure
should be given. Kodachrome is relatively
inexpensive but gives good results. It is generally
slow but gives good colour rendering with
satisfactory grain and good definition. It is
intended for daylight use but will give good
results with other light sources if a suitable
conversion filter is used.
The light source
To obtain good shots light needs to be of the
right quality for the film. To a marked degree,
the eye is able to make corrections for the
colour of light so that objects are still seen at
their ‘correct’ colour for a range of different
light sources. Film is not able to do this and
works best with a particular light source. The
colour temperature of some common light
sources are as follows: