72 PHOTOGRAPHY
Photographic Theory
The basics of photographic theory apply both to traditional
film cameras and digital cameras. To get a correctly
exposed image, the amount of light reaching the film or
the digital camera’s light-sensitive chip has to be controlled.
This is achieved by using the right combination of lens
aperture and the camera’s shutter speed. The aperture
(referred to as the f-stop) alters the size of the hole con-
trolling the amount of light passing through the lens. The
shutter-speed controls the length of time for which the
film or chip is exposed to light. Together, they affect
the total amount of light reaching the film or chip. If one
is changed (for example, to increase the depth of field –
see below), the other must be adjusted accordingly. If only
one is adjusted, such that the film or chip receives too
little light, the image will be dark or underexposed; too
much light and the image appears light or overexposed.
In either case information is lost from the image.
A third influencing factor is the sensitivity of the
image-capture medium (for film, the speed of the film;
for digital, the sensitivity setting of the chip – as an ana-
logy to film, it is referred to as the ISO number). This can
be changed by using film of different speeds, expressed
by its ISO or ASA numbers, or by adjusting the digital
camera’s sensitivity or ISO. The most common types of
film, from least to most sensitive, are 64ASA, 100ASA,
200ASA, and 400ASA (the ISO number is the same).
This affects the quality or graininess of the image. The lower
the sensitivity of the medium used, the finer-grained
the image will be, with better definition, resolution, and
clarity. As the sensitivity of the medium is increased,
‘noise’ in the form of film grain (film) and pixelation
(digital) increases. This cannot be avoided; it is a fact
of life. In underwater photography, higher, more sen-
sitive ASA/ISO speeds are usually used to compensate
for lower light levels.
In traditional photography, photographic film, which
is sensitive to light, retains the exposed (or latent) image
until developed. In a darkroom, once the film is devel-
oped, light is projected via an enlarger, through the
film ‘negative’, onto photographic (light-sensitive) paper.
This paper is then developed and results in a permanent
photographic print. The principles of digital photographyFigure 10.1 A diver sketching a late nineteenth-century shipwreck in Dor, Israel. (Photo: Kester Keighley)