Density (photography)

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Under density or density is understood in the photo technique the extent of conversion of a photosensitive material due to the exposure and development into visible gray / black (blackening) or color. For transparent photographic materials this is the optical density (extinction), for reflective material (paper image) the size is specified accordingly.

The sensitometry is concerned with the relationship between the exposure and the density achieved, by the range of density and the density curve (also gradation curve may be) described, and influenced by the development process.

Density range

The range of density is the range between the minimum (D min ) and maximum density (D max ) of a light-sensitive material, i.e. for a film the density of the carrier material ( blank film density ) or the maximum blackening.

Example values ​​for the maximum achievable density D max :

When digitizing films, the scanner must be able to resolve the low levels of brightness with maximum blackening. Consumer electronics products generally cannot do this. One constructive measure is the use of a sensor with a particularly large dynamic range , such as a photomultiplier (in the drum scanner ). An alternative is the calculation of several differently exposed scans, see multi-exposure and HDRI generation from exposure series .

Density curve

Density curve; the linear area is between points B and C.

The density curve describes the relationship between exposure and the reaction of the light-sensitive material (film, sensor). The steepness of the density curve (with a logarithmic plot of the exposure) is called the gamma value , the difference between the maximum and the minimum density is also called the range of density.

If small changes in the exposure lead to strong changes in the density, one speaks of a hard behavior (gamma value greater than 1), conversely of a soft one (gamma value less than 1). Color negative films tend to be soft, slide films to be hard. Black and white photographic paper can be provided with variable contrast (with an adjustable density curve).

Photographic material is only able to react linearly , i.e. directly proportional , to changes in exposure (more precisely, the logarithm of exposure) to a limited extent . Outside this range, changes in exposure are no longer converted proportionally (i.e. with constant gamma) into a change in density (density). Chemical films react outside of the linear range by slowly approaching the minimum and maximum density, whereby color changes can occur. There is basically a similar saturation behavior with electronic image sensors, but the dynamic range is usually "hard" cut during the subsequent signal processing (analog-digital conversion), whereby in typical situations only individual color channels are often affected. The abrupt transition from the (approximately) linear behavior (gamma = 1) to a constant maximum value and the associated artifacts are typically more noticeable than the continuous color shifts in traditional photochemical processes.

Implications for photography

Film material and sensors have a limited range of density. An exposure range corresponds to this via the density curve. Only when the subject contrast is smaller than the exposure range, the subject can be reproduced completely in all highlights and shadows. With the zone system an attempt was made to optimize the reproduction chain from the motif to the positive.

The gradation curve in digital image processing

Some software for digital image processing offers as an aid to change images with the help of a gradation curve ( tonal value correction ). A relationship between the input values ​​of the images (or individual channels) and the calculation result can be drawn up. Possible applications are the increase of the contrasts in the shadows and lights or a negative representation of the image (reversal of the values).

See also