Image quality

A photograph has certain physical characteristics that define the 'quality’ of the image, irrespective of the content. These characteristics can be assessed objectively, with instrumentation, or subjectively by visual assessment. Generally the quality of an image is determined by the initial choice of film, exposure and processing of the original.

The goal of any duplication or copying of a photographic image is to accurately reproduce the tones of the original.

Image quality definitions

Contrast

The perceived (subjective) or actual (objective) difference between two or more parts of an image in terms of tone or luminance. In assessing the image, contrast can also refer to the compression of tones between the lightest white and the darkest black, tonal compression.

Low contrast

Normal contrast

High contrast


Fig 13.1 tonal compression

Density

A measure of the 'light stopping’ or 'light absorbing’ ability. Density (D) is defined as the logarithm of the ratio of the light falling onto (incident) and light transmitted (for a negative) or reflected (for a print) from a sample.

Fig 13.2 Transmission and deriving density

D = log Io/I

D = density
Io = incident light
I = transmitted or reflected light

Densitometer

An instrument that measures the density of an image. The reading aperture is most commonly 1-2 millimeters in diameter and reads a fixed point of the image. Microdensitometers measure very small areas of a moving sample and the reading is given in a plot of changes in density over a given area.

Exposure

The act of light as a radiant energy falling upon a photosensitive material. Photographic exposure is the intensity of the light combined with the length of time the light is permitted to fall upon the photosensitive material.
Exposure is a critical step in photography. In combination with processing exposure plays an important role in tonal reproduction and image sharpness.

H = Et
 
H = quantity of light per unit area
E = illuminance or intensity
t = time

Graininess

A subjective sensation of non-uniformity in the image. It is often attributed to the individual grains of a processed emulsion, due to the very small size of an individual grain. This is not true. Due to the random nature of scatter of grains within an emulsion, including depth, they may appear to be clumped together. This forms an irregular pattern on a much larger scale than individual grains.

Granularity

The objective measure of the lack of homogeneity of the photographic image is determined from the spatial variation of density recorded by a micro densitometer. This is the measure of the small fluctuations in density. It is the granularity of a negative that gives the graininess of the print.

Luminance

A measure of the intensity of a light source. Luminance is properly defined as the luminous intensity per square metre, where luminous intensity relates to the output of a standard light source.

Magnification

The arithmetic ratio of the original object to the final object; e.g. 1:2, 1:4, where a proportional change in dimension is produced optically. It can also be expressed as a fraction: ½, ¼ etc.

Resolution

Fig 13.4 Resolution test chart

The ability to show separation between two or more elements (fine detail) in an image. Resolution of a photographic system is measured by the use of test targets that give a reading in line pairs per millimeter. Resolution in itself is not a good indicator of the quality of the image. It is but one factor that leads to the impression of a high quality image.

Higher contrast – lower resolution

Lower contrast – higher resolution

Fig 13.5 Resolution and image contrast

Sharpness

A subjective response to the combination of the many factors that are used in producing a photographic image:

  • resolving power of the lens and the film emulsion
  • focus
  • subject movement
  • camera movement
  • exposure and processing
  • subsequent printing or duplication

A poor response from any component will reduce the sharpness of the final image.