Film speed
If we use a high film speed in a photograph, for example ISO 1600, we will need less light for a proper exposure, than if we were using ISO 100. This means, at first, that when there is lots of light in the scene we are shooting (for example, under sun light) we will use a lower film speed (ISO 100, ISO 200). When there is little light (at night, or indoors) we will use a higher film speed (ISO 800, ISO 1600). In other words, if we use a high ISO (1600) we will need less light than if we use a low ISO (100).
The standard ISO, which renders an excellent quality, is ISO 100. The majority of people used ISO 100 to shoot photographs, whether they knew this or not. Nowadays, compact digital cameras generally have an automatic ISO, and the camera sets the ISO it considers necessary for the light present in the scene we are shooting.
It is very important for the person taking the photograph to control and choose the ISO. Why? Because the ISO, besides determining the proper exposure of the photograph, also has a direct effect upon the quality of the final image.
If we use an ISO lower than 100 (in film) the definition and colour saturation increases considerably, specially if we are using slide film. In the case of digital cameras, there is no noticeable change in definition but colours are saturated more.
When we are using film, the ISO will determine the definition in the image. When we use a high ISO, such as 3200, we will notice picture grain. This results in a loss of definition in the photograph. Grain are (tiny) points which appear in the image. They are actually gaps between the silver grains which form the image. The more grain in a photograph, the less definition it has. When we photograph with ISO 100, grain is not visible in the photograph, thus, there is more definition.
Despite the loss in definition, grain is visually pleasant. As a matter of fact, many photographers use films with high film speed to achieve an image with visible grain adding a personal touch.
Both photographs above are the same picture. The first one is the original picture. I shot this photograph using a black and white film in 3200. You can see that there isn't much definition in the first image. The second photograph is an enlarged version of the first. As you can see, there is a loss in definition and grain is visible. I used a 3200 film on purpose to achieve this effect.
