- Exposure latitude is the ability to continuously and faithfully discriminate differences in exposure intensity such that the full tonal range of the original scene may be reproduced
- Wide exposure latitude also provides greater safety factor for both deliberate and accidental over- and underexposure
- The use of multiple emulsion layers results in greater exposure latitude and finer grain in each layer
In order to achieve wider exposure latitude, film emulsion contains multi-layers in each color layer.
If we look at cross-sections of different types of film under a microscope, we see many different layer orders.
In a black-and-white negative film there are only two light-sensitive layers, a fast and a slow, to provide exposure latitude.
In color negative films there can be two or even three layers, fast, mid and slow for each color record, to provide exposure latitude and minimize grain.
Here we can see how the fast, mid and slow emulsion layers are used to form different parts of the characteristic curve of a color negative film.
The large crystals in the fast layers form the toe of the curve, the mid size crystals form the linear part of the curve, and the finest grains form the shoulder and Dmax.
This provides a film with low contrast and long-exposure latitude.
Therefore, when a color negative film is underexposed, most of the image tone is recorded in the toe region of the curve, the image will look more grainy.
We can also see how the use of multiple emulsion layers builds up a long characteristic curve with plenty of exposure latitude by combining three shorter curves that add together.
In simple term, using multiple layers in film emulsion is just like the gear box in a car. When capturing the shadow (low vehicle speed), the fast layer (low gear) begins to be exposed. When capturing mid tone (mid speed), the medium layer (mid gear) is working. Finally in capturing highlight (high crusing speed), the slow layer (high gear) is working.
In next topic, I will go into details about exposure, image formation and film processing.
To be continued……