Image characteristics

Capture mediums have the following characteristics: ~ Sensitivity (speed) ~ Sharpness (grain and/or resolution) ~ Contrast ~ Noise (digital sensors only).

Sensitivity

All films and image sensors are assigned an ISO (International Standards Organization) rating. This rating indicates its sensitivity to light. The higher the rating the greater the sensitivity. Most image sensors can be assigned different ratings as and when required. Films are available from 50 ISO to 3200 ISO whilst image sensors are available that can be rated between 50 and 6400 ISO. Each time the ISO doubles the film or image sensor is twice as light sensitive, e.g. a 400 ISO film requires only half the exposure of a 200 film. The 400 film can therefore be said to be one stop faster than a 200 ISO film and two stops faster than a 100 ISO film. Films are often referred to as slow, medium or fast. A film is described as being slow if its ISO is 64 or less and fast if its ISO is 400 or more. The advantage of using fast film or an image sensor rated at a high ISO is a photographer is able to use faster shutter speeds to either freeze action or avoid camera shake. The disadvantage of using faster film or an image sensor rated at a high ISO is its decreased resolution or sharpness (film only) and the increased size of film grain or digital ‘noise’.

Sharpness

A digital image is made up of ‘pixels’ or picture elements whilst the silver image on film is made up from a grain pattern of silver halides. As a film’s ISO increases the size of the grain increases whilst increased noise levels result from raising the ISO of an image sensor. The grain pattern or noise becomes apparent when the image is enlarged. As the image is enlarged it appears progressively less sharp. An image created using slow speed film or a high resolution image sensor will enlarge to a greater physical size before the sharpness becomes unacceptable than if the same image was created using a fast speed film or lower resolution image sensor. Reduced grain size or low noise levels are strong selling points for fast films and high ISO image sensors.

Contrast

Digital image sensors have a ‘latitude’ similar to transparency film when shooting JPEGs in camera so great care must be taken to protect images from loss of detail resulting from excessive contrast. The photographer should save images using the Raw format in high-contrast situations. As the ISO rating of film increases the contrast decreases. The slower the film, the higher the contrast. As film is ‘pushed’, however (increasing the recommended film speed and processing time), the contrast increases. This is a result of the processing and not the film speed used for exposure. High-contrast film is not easy to use with high-contrast subject matter. If the photographer is not skilled with this combination, highlight and shadow information may be lost.