Digital Photographer's Software Guide - Bertemes - Net

21
High Dynamic Range
Dynamic range is the scale of values from the darkest dark to the brightest bright. All
efforts to represent the real world, whether in painting, film, or digital photography,
have to reduce the dynamic range to fit the capabilities of the medium. For example,
reflective paper is bound to have a more limited dynamic range than a light-emitting
screen, even if the manufacturer adds a lot of whitener to it. Most modern displays have
a contrast ratio of around 1000:1 or 2000:1 (some are higher), but new displays developed
by BrightSide Technology have a very high peak luminance (greater than 3,000
candela/m2) and a contrast ratio better than 200,000:1. A high dynamic range (HDR)
image of a landscape on this type of screen looks as if you could step into it. It is that real.
In basic HDR photography, the photographer takes a series of shots of the same scene,
using a tripod for absolute steadiness and exposing separately for the highlights and
shadows. This can also be done automatically by bracketing if there is any movement
in the scene. The next step is to use software to blend the images together in a process
called contrast blending.
Where true HDR differs from normal digital photography, however, is in the way it
encodes the greatly increased number of tonal levels. It uses a 32-bit file format to specify
floating-point numbers that can represent millions of levels if called upon to do so.
Digital cameras are not yet able to bracket a sufficient number of exposures to make full
use of this enormous range and no output medium, except specialist displays like those
mentioned previously, can represent HDR adequately. Hence the brightness levels have
to be squeezed together by “tone mapping,” available in HDR packages and in
Photoshop CS2 and CS3.