High Dynamic Range (HDR) Photography or Imaging
Makes a Scene Look Exactly Like What the Eye Sees
When you take a photograph from a digital camera of, say a beautiful pastoral scene, and straightaway view it on the computer, or in print, what you see would be a subdued image of the scene, removed to some extent from the reality that your eyes beheld while shooting. This is because the digital image is not able to capture in full the quantity and distribution of natural light in the actual scene. In fact, no camera can capture the full tonal details of the actual scene in one exposure.
Light distribution in nature is dynamic, which means that the light falling on any object has different and changing intensities which cannot be faithfully recorded by the camera lens. But we can capture the total range of light, (the tonal details) through any camera in one way : by taking a number of identical pictures of the scene with different shutter speeds, and then merging all of them to make a single image. That single image will encompass the whole range of light at the scene, thereby bringing it close to what the eye sees directly. Thus, the tonal details of a scene cannot be captured in a single exposure. It has to be a multi-exposure and merging job.
The technique by which the entire range of light distribution (or tonal details) of a scene can be captured faithfully in a photograph is known as High Dynamic Range Photography or Imaging (HDR Imaging). Through HDR imaging the photographer captures the actual light distribution in a scene to make it realistic. This merging of images into a single image is done by means of the Photoshop CS2 software. The "merge to HDR" feature of the software brings together a series of exposures of the identical scene into one image containing the tonal details captured in each exposure. Such a "merged" final picture will have the depth and tonal quality of the real scene.
For creating an HDR image, at least 3 exposures will do, though for maximum accuracy, 5 or more exposures are recommended. More the exposures, more the quantum of light that the camera can convert to digital values to enhance the blending of tonal details. The Photoshop CS2 creates a separate HDR file for each image, recording the shutter speed, aperture and ISO setting in each exposure so that the light intensity in them can be assessed (as it may vary from exposure to exposure). The software creates an HDR file by using 32 bits to determine each color channel, instead of the 8 or 16 bits which lie in the Low Density Range.
Though the HDR technique helps us get realistic tonal qualities in photographs, it should be used with great care and skill because overdoing the merging job could distort the image of the scene photographed. There can be no substitute for good lighting, and nothing can come close to the High Density Range of the real scene which the eye naturally perceives. It must also be remembered that improvement of tonal details of a photograph also extracts a price by way of reduction in the contrast among the tonal values. Like everything in life, any technology available for betterment has always to be used with balance.