Conversion Of Speed Systems; Colour Film; Black-And-White Films - Kodak retinette User Manual

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Table Of Contents

Table of Contents

The above speed figures are based on the latest ASA Standard for film speeds (and on the BS and DIN Standards

under revision). These figures, when used on the exposure meter, give minimum correct exposures, to make the

most of the versatility of the film and of the image quality. They are also the figures quoted by most film

manufacturers. Sometimes films are, however, still rated according to earlier standards which in effect incorporated

a generous safety factor against under-exposure -- by the simple process of over-exposing films about 100 per cent

(well within the exposure latitude of most black-and-white films). So you may come across films apparently only half

as fast as others of similar type, because of this difference in ratings. The table on page 42 indicates the current

film speeds to be used with the exposure meter, even if the film packing gives a lower rating.

This applies to black-and-white negative materials only; speed rating methods have not changed for colour films.

GRAIN, The picture in the emulsion is formed by silver grains. To the naked eye they form a compact. dark mass,

but under the magnifying glass or microscope the separate clumps of grains are visible. Obviously, if the grain of a

film is coarse, it will soon become visible on moderate enlarging: and the finer the structure of grain, the more

enlarging will it allow without showing any unpleasant granular effect in the print. As a rule, it can be said that the

faster the film, the coarser the grain and vice versa. The grain can, to a certain extent, be influenced by

development (fine-grain development), correct exposure, paper, etc.

LATITUDE. Latitude is the ability of the film to yield usable negatives, even with a certain amount of under- or over-

exposure. Films with wide latitude are likely to have less resolving power, causing loss of definition.

The negative material has a number of additional properties which help towards good results. There is, e.g., a

special "protective coating", a hardened gelatine layer on top of the actual sensitive layer which protects against

scratches. The base is tinted -- as a rule grey -- in order to avoid reflection of the light coming through the emulsion

on to the film-back and thus causing halation.

Colour Film

Beside the films mentioned which reproduce the world around us in black and white, colour films enable us to

photograph in natural colours. There are two types.

The first, colour negative film, produces negatives in colour. These resemble ordinary negatives -- the dark parts of

the subject are light and vice versa -- and in addition the colours are reversed. Thus blues are yellow or brownish,

reds are blue-green, and greens are reddish.

These colour negatives are then printed on a similar kind of material to give colour prints. We can also use these

negatives to obtain black-and-white prints.

The second type of colour film, known as reversal colour film, produces positive colour transparencies on the film

exposed in the camera. These transparencies can then be viewed by transmitted light or projected through a

projector.

While colour negative film is suitable for daylight and artificial light, colour reversal film is made in two separate

emulsions, one balanced for daylight, the other for artificial light.

EXPOSING COLOUR FILM. The exposure latitude of colour film is small. It is therefore important to ascertain the

exposure time accurately with a reliable photo-electric meter. Under-exposed and over-exposed films not only

produce dense or weak results but also the colour values are distorted. Over-exposure produces pale, diluted colours

under-exposure gives hard, deep colours.