HDR Images Rescue Details from Shadows
One of the things high dynamic photography is meant to do is bring details out of shadowy areas. The photo in this figure looks west at sunset. The sky is reasonably light, and you can make out the clouds and the glow from the sun. These elements also reflect off the water of the river in the foreground.
Unfortunately, the trees, riverbank, and details of the building are all in shadow. The camera can’t capture enough dynamic range — the total range of light in a scene — to accurately represent it. Standing there, you would see that the actual scene was much brighter. The disconnect between reality and the photograph is that digital still cameras do not see scenes the way our eyes do.
The reasons cameras have limited dynamic range are many. Camera manufacturers are currently unable (due to the fact that not everything is possible) or unwilling (they have to stay in business so not everything that is possible is practical) to cross scientific, technical, design, and manufacturing barriers, including
The bit depth the sensor uses to store data
The inherent noise level of the camera system and how it affects the sensor’s ability to measure light
How digital systems simply react to light differently than our eyes do
The effect, more often than not, is that a camera compromises when it measures the exposure of a scene. It has to. In this case, rather than make the sky too bright, the camera made it just bright enough and relegated the rest of the scene to the shadows.
This figure shows the same scene in HDR. Notice the huge difference across the photo, particularly in the previously dark areas. The trees are now clearly green. The sky and clouds have more definition and the building has discernable details. Much better!
The secret to creating the new look is twofold. First, HDR photography captures a much wider range of exposure information using brackets. This gives the software much more information to use. Highlights that are normally too bright are captured so they aren’t overblown. The same goes for shadows.
In software, you tone map the source material (which has too wide a range of brightness to display or use as a standard image), squeeze it so it fits in a standard image, and make creative decisions that define the relative brightness of parts of the scene.
After this, you can make other adjustments and enhancements (such as brightness, contrast, color, noise, sharpness, recomposing, cropping, and resizing) in a photo editor such as Photoshop Elements.
This example illustrates a few interesting points:
Losing the low end of dynamic range: Digital cameras often lose details in shadows. This is caused by the camera’s inability to capture the full range of light in a scene. This range is called dynamic range.
Compromises: Cameras make compromises on what to expose well and what to let go. You can help the situation by metering the scene and subject correctly — but scenes with a wide dynamic range can’t be faithfully captured by today’s cameras.
Rescuing details from shadows: HDR photography allows you to pull meaningful information out of shadows so the areas are brighter.