Some film characteristics are described below:
a. Cross-section of Film
A cross-section of film, examined under the microscope, would look
The thickness of the emulsion varies from 5µm to 20µm,
depending on the type of film and its sensitivity. Films with a
low ISO rating have a thinner emulsion than films with a high ISO
If we look at an exposed and developed film under the microscope
we see the dark specks of silver. These specks are called the 'grain'
of the film and the size of the film grain depends on:
The film speed: slow films have smaller, same sized grains and
fast films bigger grains of varying sizes.
The developer used to develop the film.
The method of agitation and total 'wet time' of the film.
A narrow band of electromagnetic waves with wave lengths from 400-700
nm is seen by the human eye as light. The eye is the most sensitive
to wavelengths of 570 nm, that of yellow-green light. Reds and blues
are perceived as dark colours.
This is shown in the figure below.
silver halides (silver salts) used in photographic emulsions are
only sensitive to ultra-violet and blue light. To make film 'see'
colour like the human eye it has to be sensitized for other colours
as well. By using colour couplers, film is sensitized for green,
red and even infrared. Emulsions without any colour couplers are
called as 'ordinary' or 'non-colour sensitive'. Films that are sensitized
for green are called 'orthochromatic' and those sensitized for green
and red are called 'panchromatic'.
When light falls on a developed negative, part of the light will
be transmitted through the negative and part of the light will be
reflected or absorbed by the negative. The relation between the
light incident on a piece of exposed film and the light transmitted
through that piece is called the opacity of the film. The log10
of the opacity is called the density of the film. Density is used
as a measure of the 'blackness' of a tone on a processed negative.
Black parts of a film have a high density and clear parts a low
The range of grey tones an emulsion is can produce, from complete
transparency to darkest black, is called the contrast of the material.
A small range of grey tones is called 'high contrast' and a large
range of grey tones is called 'low contrast'. Plotting the density
of different grey tones against the necessary exposure, results
in the following graph:
In the 'high contrast' material a few big 'steps' are taken to go
from A to B. But in the 'low contrast' material many small steps
are taken from A to B. The gradient of the straight part of the
curve is called the 'gamma' or contrast index.
Negative contrast is inherent in the film material, but can be considerably
influenced through exposure and development. Films used for pictorial
photography are normally developed to a contrast index of 0.6 to
The exposure index
The sensitivity of an emulsion to light is called the 'speed' of
the film or paper. Films with a high sensitivity are called 'fast',
films that require more exposure are called 'slow'. To make comparisons
between films and papers possible, they are given a 'speed rating'.
There are two standards:
Arithmetic: The number indicating the speed is doubled when the
Logarithmic: For each doubling of speed, the rating is numerically
increased by three.
Both ratings are given on any film cartridge under the new ISO
(International Standard Organization) rating and are written as
follows: ISO 100/21 or ISO 400/27. In the table below some of
the old ASA (American Standard Association) and DIN (Deutsche
Industrie Norm) values are given as well as the new ISO.
The effective film speed of an emulsion is reduced for exposures
longer than 1 second or shorter than 1/1000 s. This results in underexposure
even if the exposure meter reading was adhered to. Possible corrections
can be looked up in the technical information on the film available
from the manufacturer. The adjoining graph can be used as a starting
point to calculate the corrected exposure time for exposures longer
than 1 second.