Monday, June 8, 2009

HDR (High Dynamic Range Imaging): An Introduction

HDR Image of Independence Hall, Philadelphia, PAHigh dynamic range imaging (HDR) is a set of techniques which allows a photographer to produce a photograph displaying a greater dynamic range of luminances between light and dark areas, of  the scene to be photographed, than normal digital imaging. The idea behind HDR is to be able to accurately represent the wide range of light intensity levels found in the real world ranging from direct sunlight to shadows.

OK, so that’s what HDR is, but what we’re really seeing when we see an HDR image on a monitor, or in an HDR print, is a tone-mapped image of an HDR composite image.

Tone mapping is a technique used to process an image where we map a set of colors to an alternate set, to approximate the appearance of HDR images in media with a limited dynamic range, such as computer monitors, projectors, and printers. Each of these devices, and our digital cameras have a limited dynamic range which is inadequate to reproduce the entire range of light intensities present in many natural scenes which the human eye can discern.

In HDR/Tone Mapping the problem of contrast reduction from the scene to what can actually be displayed is handled, while preserving the image’s details and general color appearance.

So that’s the technical definition, but if you’re not familiar with HDR, I know you’re asking what the heck does that mean.

Independence Hall HDR ExampleWe often, as photographers, especially on bright days, come across a scene we’d like to photograph which has light intensities ranging from dark shadow to very bright whites. The photograph above is an HDR rendering of Independence Hall in Philadelphia, PA. Please note in the photo, that throughout the range of luminances (light intensities) the scene’s detail is preserved, from dark through light.

Take a look on the right. The three photos you see show the range of luminances which make up the HDR composite photo at the top of this article. In each photo, some detail has been lost.

When creating the final photo, I used the three photos on the right, plus two more, one exposed between the dark and center exposures, and one between the center and light exposures.

In order to create the five exposures I put my DSLR on a tripod and bracketed the exposure. I started with an exposure for the brightest light in the scene and kept increasing the exposure by decreasing shutter speed, until I had the right exposure for the darkest part of the scene.

The f/stop and iso remained constant for the photos which were taken within the span of just a few seconds. I set my white balance manually to the correct color temperature of the scene using an ExpoDisc.

By making these exposures, each one keeps the scene's detail for which the exposure was made, from dark to light.

To get the best results from your HDR processing, your exposure sequence must include photos which correctly expose highlights, and photos which correctly expose shadows. Exposing the shadows properly is especially important to avoid noise showing in the processed HDR image.

Then it’s off to the digital darkroom. I’m currently using Adobe Photoshop CS4 Extended for my general photograph editing and retouching. For HDR work, I use Photomatix Pro from HDRsoft, which integrates with Photoshop. I use the stand alone version of Photomatix, not the Photoshop plug-in, as it’s more powerful.

There are two steps involved in creating and processing HDR images:

In step 1 the software merges your photos taken under different exposure settings into a single 32-bit HDR image. Because of its high dynamic range, the 32-bit HDR image will not display properly on conventional monitors.

In step 2 the software processes the 32-bit HDR image via its Tone Mapping tool. Tone Mapping will reveal the dynamic range captured in the HDR image and produce an image that can be properly displayed on conventional monitors and can be properly printed.

Once the HDR/Tone Mapping processing is complete, the composite photo is ready for final editing in Photoshop.

You can surmise from the 3 photos on the right, the luminosity range limit of the DSLR’s sensor. Clearly the range of luminosity in the scene is well beyond the capability of the camera’s sensor. It’s clear that without using HDR to create the photograph of Independence Hall with such a large range from dark shadow to bright white in bright sunlight, the photo could not include all the detail of each tonal area of the scene which our eyes see.

Without the use of HDR, I would have had to use the middle exposure, which isn’t a bad snapshot, but doesn’t include the richness of the final composite photograph.

1 comment:

Herb C. said...

Ned, you are the first person, and the first blog to explain HDR to me in a way I could understand.



Post a Comment