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Film
by
Peter Scoones
Reproduced
from in focus 10 (June
1985)
There
are several different types of film available in 35 mm or
larger format, and each type has advantages and disadvantages.
The choice within each type is greater for 35 mm than large
format.
Black
& White Negative Film
Black
& White negative film is available in varying speeds.
These may be classified as slow (up to 50ASA), medium (50
- 200ASA), fast (200 - 400ASA) and ultra fast (over 400ASA).
Slow film has fine grain (and hence high definition), high
contrast but low latitude. (Latitude is the amount by which
a film can be over or underexposed and still produce a reasonable
image). Fast film has a coarser grain (and hence lower definition),
lower contrast, but more latitude. Film speed is determined
by the minimum exposure required to change the silver in the
film emulsion to a latent image. This minimum exposure is
the threshold, exceed it and the film is exposed. Fast films
are hypersensitised either chemically or by flashing a bright
light of known intensity for a brief known period to just
below the threshold i.e. not quite enough to produce a latent
image.
Exposure
is controlled by time and light intensity. There is generally
a linear relationship between them i.e. the more light the
less exposure and vice versa, although this relationship breaks
down for very short or very long exposures. The exposure controls
density.
Development
changes the silver in the emulsion to metallic grains, the
faster the film the larger the grains. Development controls
contrast. Film speed Is not affected by development. When
you uprate or push a film, the Film speed stays the same,
but the contrast of the resulting image changes. So if you
uprate say a 100ASA film by 1 stop to 200ASA. you are actually
underexposing that film by 1 stop. When developing that film
you increase the development time (by 33% in this case) mid
consequently increase the contrast. Sow underwater photographers
deliberately underexpose their films slightly to increase
density and then over develope slightly to increase contract.
Colour
Reversal Film
Colour
reversal or slide film is available at similar speeds to Black
& White negative film. but only has a latitude of about
1/2 stop. There are many types available and all strive for
neutral balance, though few acheive it. The emulsions used
have a tendency to favour certain colour shifts i.e. the old
Ektachrome 64 was always regarded as slightly blue. The processing
and conditions affect each film type differently. They can
be pushed or uprated but care should be taken. Kodachrome
is the moat stable and has the highest definition. It is essentially
a black & white film. the colour being introduced at the
processing stage. Professional film differs from others in
that it is intended to be stored under cold conditions prior
to use and then processed immediately. Other types of film
have a longer shelf life and although they degrade, do so
much slower than professional films.
Colour
Negative Film
Colour
negative film is available in speeds from 100ASA to 1600ASA.
It has a very much greater latitude than either black &
white negative or colour reveral film, about 5 stops. As with
colour reversal film there are many types available, All are
very similar and there is little to choose between them. Fuji
has the most contrast and the most critical exposure. Kodak
is middling and Agfa is the safest. Colour negative film can
be used for colour prints or transparencies and black &
white prints.
Special
Films
In
addition to the standard colour negative, colour reversal
and black & white films, special films are available for
duplicating and printing and for infra red photography. Unlike
standard films you will need to go to a specialist suppliers
in order to buy them.
Reproduced
from in focus 10. June,
1985 |
