photosensitive material

Photosensitive Material – B & W


The word photography means “ writing or drawing with light” a phrase that conveys both the creative & the chemical nature of the photographic process.

A camera is simply a device for bringing together a sharp image with the help of the light reflected from a scene & allowing it briefly to touch a film material so sensitive that the light leaves a trace which can be finalised into a finished picture.


To form an image on film requires a correct exposure which is a combination of aperture (Opening) & shutter speed (Time).

The modern camera uses a lens, a variable size opening or aperture & has a shutter that allows light in for the required amount of time, which the photographer can control.


Characteristics of Film

1. Emulsion2. Colour Sensitivity3. Film Speed4. Graininess5. Gradation6. Exposure Latitude


Emulsion

1. An emulsion is a light sensitive material coated on the surface of the film.2. It is made of microscopic crystals of silver halides. These include silver bromide in large proportion, silver chloride & silver iodide in smaller proportions.3. When the photographer opens the shutter, light from the subject begins to act on the emulsion coating on the film that contains these crystals of silver halides.


Emulsion

4. These salts of silver are extremely light sensitive & darken instantaneously when exposed to light.

5. The light triggers a chemical change in the salts so that they start to form microscopic grains of black silver.

6. An image thus formed on film is called the latent image.


Emulsion

7. Latent image is defined as an invisible image recorded on photographic emulsions as a result of exposure to light.8. The film is then treated with appropriate chemicals to convert this latent image into a visible one. This process is called developing.


Colour Sensitivity

There are two types of films

1. Panchromatic

2. Orthochromatic


Colour Sensitivity

Panchromatic film is a term used to describe

B & W photographic emulsions sensitive to

all colours of the visible spectrum & to a

certain amount of ultra-violet light. This film

requires to be processed in total darkness.


Colour Sensitivity

Orthochromatic film is a term used to describe B & W photographic emulsions sensitive to the blue & green regions of the spectrum but insensitive to red & orange light. Hence this film can be processed in red or orange safe light.


Film Speed

The degree of sensitivity of a film to light is known as film speed.

It is expressed numerically for the purpose of exposure calculations.


Film Speed A film that is less sensitive to light & require more light to expose is called slow film or low speed film.

A film that is more sensitive to light & which can be exposed in low light is called fast film or high speed film.


Film Speed

Different units given by different photographic

companies in which film speed is measured :-

DIN – DEUTSCHE INDUSTRIE NORM

ASA – AMERICAN STANDARDS ASSOCIATION

ISO – INTERNATIONAL STANDARDS

ORGANISATION


Film Speed

DX – Coded Cassettes

DX coded films when loaded in the camera with

DX coded facility (sensors), the film speed I

automatically set on the camera.

DX – coded film cassettes have a chequer board

pattern of squares which the camera sensors read to

automatically set the film speed on the camera.

Exposure Table with different film speeds

Sr. No. 100 ASA 200 ASA 400 ASA

1 f/8 1/125 f/11 1/125 f/16 1/125

2 f/11 1/60 f/16 1/60 f/22 1/60

3 f/16 1/30 f/22 1/30 f/32 1/30

4 f/5.6 1/250 f/8 1/250 f/11 1/250


Graininess

1. Granular texture appearing to some degree in all processed photographic materials is called grain.

2. It is because of this granular texture which appears in the negative image, cannot be magnified beyond a certain limit.

3. In B & W photographs, the grains are minute particles of black metallic silver that constitutes the images.


Graininess

4. The faster the film, the coarser the texture

of the grain.

5. The slower the film, the finer the texture of

the grain.

6. Slower films are used to make big or life

size enlargements.


Graininess

Guidelines to achieve fine grain negative

image :-

1. Use slow speed film.

2. Do not over expose.

3. Do not over develop.

4. Use fine grain developer.


Gradation1. Gradation is described as the range of contrast or tonal variation in a photographic image.2. An image that has many different intermediate tones between the darkest & the lightest part is described as having a soft gradation.


Gradation


Gradation

3. An image that has very few variations

between the light & the dark parts is said

to have a hard gradation.

4. Gradation depends on the speed of the

film. Slow films produce hard gradations

& fast films produce soft gradation.


Gradation5. A film that gives the image in various tones of grey resembling the light & shade of the subject is called a normal contrast film. This is a general purpose film.6. A film that records images in total black & white, without any middle tone, is called a high contrast film. Such a film is used for graphics & line work.


Exposure Latitude1. The degree by which the exposure can be varied & still produce an acceptable result.2. In other words, it is the tolerance of a film emulsion to record satisfactorily & give acceptable prints even if the exposure is not exactly correct.


Exposure Latitude

3. B & W films have more latitude than colour

films.

4. Faster films tend to have a greater latitude

than slower films.

photosensitive material – b & w

Documents

film material

light sensitive material

slow film

fast film

film speed4

low speed film

high speed film