In photographic terms, dynamic range refers to the ratio between maximum and minimum measurable light intensities. However, the concept must be regarded somewhat differently for subjects, input devices such as digital cameras and scanners, and output devices such as computer monitors and printers. The human eye, computer monitors and printed images all potentially exhibit different dynamic ranges for the same scene. When images are transferred from one device to another, dynamic range may consequently change. In the case of a computer monitor, brightness can not exceed the maximum intensity of a typical pixel (pixel saturation). In the case of a paper print, no part of an image can be rendered brighter than the surface of the media used. The human eye also has its own dynamic range.
Although maximum and minimum measurable light intensities might be described as the equivalents of white and black respectively, such absolutes are not normally encountered in the real world. In practice most scenes incorporate a range of brightnesses arising from variations of both source intensity and subject reflectivity. Indeed scenes including large variations of subject reflectivity, such as those incorporating both strong reflections and matte black objects, are likely to feature greater dynamic range than those with large variations of intensity of incident light only. Dynamic range can often exceed that of any digital or film camera, or any other recording medium. Exposure must be carefully chosen to make the best of such difficult circumstances, but some form of contrast reduction may also be necessary to achieve a satisfactory image.
The importance of light intensity when measuring dynamic range makes accurate intensity measurement vital. Light intensity can be described in terms of incident and reflected light, both of which contribute to the dynamic range of a scene. The diagram below shows approximate values for incident light, or illuminance, for light sources to which everyone is exposed in everyday life. Note that the logarithmic scale extends from 0.001 to 100,000 candelas per square metre - a vast range of intensities. It is consequently easy to understand that any unevenly illuminated scene may exhibit a huge dynamic range.
In practice, the dynamic range of a digital camera is limited not only by the maximum intensity of a typical pixel but also by the black level - a dark tone where texture is no longer discernable. This black level is determined to a large extent by the accuracy with which each pixel can be measured, and hence by noise. This is the basic reason why dynamic range is generally greater at lower ISO ratings and, importantly, when using cameras with larger individual pixels (and hence larger sensors in most cases) which feature lower noise levels. The precise effective dynamic ranges of digital cameras, and in particular their sensors, tend to be closely guarded by commercial confidentiality. However, in broad terms, the dynamic range of a camera may be described as the ratio of the maximum and minimum measurable light intensities. This ratio is typically expressed as a number of f-stops or "eV". A dynamic range of 9 f-stops (or 9 eV) is equivalent to a contrast ratio of 512:1 (since 29 = 512). Similarly, a dynamic range of 12 f-stops (or 12 eV) is equivalent to 4,096:1 (since 212 = 4096).