The Film


The silver salts, silver bromide, silver chloride and silver iodide are light sensitive and discolour when they are exposed to light. In a photographic film these salts are suspended in gelatine to form the photographic emulsion. In earlier days the emulsion was coated onto glass plates and later onto celluloid. Unfortunately celluloid burns easily and today a nonflammable base of acetate or polyester is used to give us the "safety film".

When a photographic emulsion is exposed to light, the silver halides change to photo-activated silver and bromide atoms. The silver is said to be triggered. The information contained in the emulsion is very small, not visible and referred to as a 'latent image'. During development more silver halides are reduced chemically to silver and the 'latent image' is intensified and becomes visible. The remaining silver halides are removed during the fixing process.
The parts of the film that receive a lot of light will contain much silver after development and appear dark. The parts that did not receive any light will remain clear. The film will be dark were the subject was light and light were the subject was dark - in other words: we have a negative image on film. By exposing a photographic paper or film through this negative we get a paper or film positive.
 

 
1.

Film Characteristics
Some film characteristics are described below:
a. Cross-section of Film
A cross-section of film, examined under the microscope, would look like this:

 
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 rating.
 
b. Grain
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.
c. Colour sensitivity
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.
 
 

The 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'.

d. Density
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 density.

e. Contrast
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 0.8.

f. 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 sensitivity doubles.
  • 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. 
ASA
25
50
100
125
200
400
800
DIN
15
18
21
22
24
27
30
ISO
25/15
50/18 100/21 125/22 200/24 400/27 800/30
 

g. Reciprocity Failure
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.
 

2. Film Processing
a. Chemicals
All chemicals used in processing photographic films and papers should be handled with care! Always read instructions on the labels and take the precautions advised. Mix chemicals in well ventilated areas, keep your hands out of the solutions and wear protective gloves if necessary. Do not eat, drink or smoke in the darkroom.
  • Developers
  • Developers are available in powder form or as a liquid concentrate. Make up powdered developers exactly as instructed and let stand for 24 hours before using them. Never mix part of a batch and always add chemicals in the sequence the appear in the recipe. Used developer may be poured down the drain.
  • Stop-baths
  • Commercial acid indicator stop-baths should be mixed according to the instructions on the label. They normally have a light yellow colour that changes to purple when the solution is exhausted. You also can mix your own stop-bath of 2% glacial acetic acid.
  • Fixers
  • Most fixers are liquid concentrates. Mix according to instructions and note what fixing times are given for the dilution used. To test a fixer, place a piece of undeveloped film into the fixer and note the time it takes to clear. Double that time will be the correct fixing time (tf) for the film.
  • Note: Never pour exhausted fixer down the drain. Store it in a bottle and arrange for silver recovery firm to collect your exhausted fixer. They will even pay you for it.  

b. Development Procedure

Prepare the developer, stop bath (2% - 3% acetic acid) and fixer according to the manufacturers instructions and check their temperatures.
Use a water bath to bring the temperature within the 20°C - 25°C range. Note developing and fixing times. If the temperature is higher or lower than the recommended temperature, find the correct time from the chart on page 10 and set the timer (td).
In complete darkness insert the exposed film into the dry spiral. Place the spiral into the tank and close it properly.
You may then continue in the light.
Pour the developer quickly into the tank and start the timer. Close the lid of the tank and knock the bottom twice to get rid of any air bubbles sticking to the film.
Invert the tank, i.e., turn it upside down and then right side up, 4-6 times in the first ten seconds of every minute for the rest of the developing time.
Pour out the developer ten seconds before the end of the developing time.
Pour stop bath into the tank and close the lid. Invert tank 6 times, let stand for 15 seconds, invert 6 times and pour out the stop bath.
or
Rinse film in running water for 1 minute.
Pour fixer into tank, close lid and for 30 seconds keep inverting the tank continuously, let it stand for 30 seconds. Then turn it upside down 6 times every minute for the rest of the fixing time.
When the fixing time is over, pour the fixer into its storage bottle and wash the film using one of the following three methods.
1. Place the tank under a water tap and wash the film in running water for 20-30 minutes. The water temperature should not be below 18C.
2. Wash the film in running water for 1-2 minutes as in 1. Pour water out and pour a hypo clearing agent or wash aid into tank. Agitate for recommended time. Pour wash aid into its storage bottle and wash film in running water for 5 minutes.
3. a. Fill tank with water and invert it five times. Pour water out.
b. Refill tank and invert it ten times. Pour water out.
c. Refill tank for the third time and invert twenty times.
Note: Use 500 ml per 36 exp. film. This method is recommended by Ilford and does not necessarily apply to other makes of film. Kodak's T-max films definitely have to be washed longer.

When you have finished washing the film, rinse the film in a wetting agent (diluted according to instructions) for 1 minute and hang it up to dry in a dust free place.
Thoroughly wash the tank, spiral and all other containers used and allow to dry before storing.
Film development is a chemical process and therefore temperature dependent. The higher the temperature, the shorter the processing time. If your temperature differs from the temperature given in the table, calculate the new development time from chart below.

Time-temprature development chart



1. Find the point representing the time at the temperature given in the table.
2. Follow the diagonal line corresponding to this time to the point where it cuts the horizontal line representing the temperature of your developer.
3. Draw a line straight down from this point and read off the new development time on the horizontal axis of the chart.

The following table gives developing times for some films and developers at 20C. These times are for normal contrast. If you use a diffusion enlarger, increase the time. Agitation should be 4-6 inversions in the first ten seconds of each minute, except for Rodinal -continuous agitation for the first minute, then two inversions every thirty seconds for the rest of the development time.
These times are only a starting point and each photographer should do his own tests to decide on the best results. For consistent results it is advisable to use processing times that are longer than 5 minutes.

Film - developer table
 
 
Time in minutes at 20C
Film
ISO
rating
Rodinal
(1+50)
ID11 or D76
stock
Ilfotec HC
(1+31)
HC110
Dil. B
Tmax
(1+4)
Agfapan 25
25
6-9
   
5
5½
Agfapan APX 25
25
     
6½
 
Agfapan APX 25
50
       
5½
Agfapan APX 100
100
     
7
6-7
Agfapan 400
400
   
6
   
Agfapan 400
1600
     
7½
8
Fuji Neopan 100 SS 
100
5
6
 
6
5
Fuji Neopan 400 SS
400
 
7½
 
4
 
Fuji Neopan 400 SS
1600
 8
     
7
Ilford 400 Delta
400
 
6
6
6
5½
Ilford FP4
125
11½
6½-7½
3½
5½-7
7
Ilford FP4 Plus
125
 
6
6
5
4½
Ilford HP5
400
9
7½-8½
6½
6
 
Ilford HP5 Plus
400
 
7½
   
6½
Ilford HP5 Plus
800
12
10½
 
8
8
Ilford Pan F
50
 
6
 
6
4
Kodak Plus-X pan
125
 
6½
4½
4½-5½
 
Kodak T-max 100
100
8
9
5
7
6@24
Kodak T-max 400 
400
8
8½
 
6
 
Kodak T-max 400
800
 
8
   
7
Kodak Tri-X pan
400
 
7-8
6½
6½-7½  
Tura P150
100
8-9
9-10
 
9-10
8
Tura P400
400
11-12
10-11
 
8-9
7
 
 

 

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