HDR, Understanding The Coop

Brenton W. Cooper

New Member
Howdy-do,
My interest in High Dynamic Range Imagery (HDRI) all started when I saw a photograph on fellow photographer's website. It was panoramic shot of a landscape, with a early evening sky. Allow me to paint in words what I saw and felt. The sky colors were rich and detailed and so was the landscape. The sun was setting in the lower right hand corner of the frame, which usually mean blown out highlights. The detail in the clouds right above the sun were remarkable. There was detail everywhere. I said to myself: "How in the heck was this photographer able to get vibrant colors and sparkling green grass, not to mention the details in the distant mountains while almost looking directly towards the sun?" Sure, maybe he masked the sky in Photoshop using the selection tool and then brought back some color and detail, but to these extreme! This is different! Then I thought, wow, I could maybe do the same thing to the great landscapes that I have on the Mojave Desert and the Sierra Nevada Mountains. Maybe even apply this technique to street scene that I have of Reno and Las Vegas. Normally the sky becomes severely blown-out, over-exposed when a person exposes for the foreground in a highly contrasty scene.




On this photographer's site, he briefly mentioned a process called "High Dynamic Range Imagery," and he himself stated that he was able to enrich the details by blending different exposure of the same thing together into one image. But that was it...he didn't explain the process. But I thought, "yeah the old standard of bracketing exposures from the film days. During the film days and even today when I choose to photograph with film I'll bracket my exposures and then when I'm eye-balling them on the light-table I'll cull out the poor images and use the best of what I have. However, I never did throw the over/under exposed negs or positives away, I would keep them because one never knows when they might come in handy. It took me awhile myself to gather enough information on HDR so I would know how it's done, and what I needed to do, or to, purchase what software I needed to get myself started. But now, everywhere you can pick up almost any photography magazine without spotting an article on HDR. And because it is digital, I still bracket all the time. It's digital, so who cares. Auto-Exposure bracketing has been around for quite awhile. And that is what I do. I bracket everything. But, for what it's worth...here in the following days I'll explain what I do..., that is of course, gentle reader if you don't mind?
 
What is High Dynamic Range? It means a broad range of light that can be measured and recorded by a single exposure or many exposures and then blended into a final exposure using HDR designed software.


In image processing, computer graphics, and photography, high dynamic range imaging (HDRI or just HDR) is a set of techniques that allow a greater dynamic range of luminance between the lightest and darkest areas of an image than current standard digital imaging techniques or photographic methods. This wide dynamic range allows HDR images to more accurately represent the range of intensity levels found in real scenes, ranging from direct sunlight to faint starlight.http://en.wikipedia.org/wiki/High_dynamic_range_imaging#cite_note-0
The two main sources of HDR imagery are computer renderings and merging of multiple photographs, the latter of which in turn are individually referred to as low dynamic range (LDR)[2] or standard dynamic range (SDR)[3} photographs.
Tone mapping techniques, which reduce overall contrast to facilitate display of HDR images on devices with lower dynamic range, can be applied to produce images with preserved or exaggerated local contrast for artistic effect.

Here is a typical approach to HDR photography: By taking multiple images of the same scene at various exposure values using AEB(auto-exposure bracketing), the images can be blended together(cooked) in software designed to process these images. By photographing numerous images, between three to nine exposures of the same scene at various exposures ranging from over and under exposures up to two or three stops on either side of a normal exposure, the HDR merging software of these images recaptures the brightest and the darkest details that would be lost through just using one exposure. The HDR software takes all of the merged images that have been chosen and creates it's own new image (photo file) as large as a 32-bit images that hold all the information needed to express the entire light range. Then a scond process called tonemapping transfers the 32-bit file into a viewable range of either 8-bit and/or 16-bit. This second process is called "tonemapping." This final image file produces an extented range of colors and details through the scene in both the highlights and in the shadows.

This tradition approach to HDR requires a sturdy tripod to keep the camera stable because it is imperative that the images line up exactly, overlapping one another. Ideally the subject matter is also stationary so that no ghosting occurs. It is preferable to have absolutely no movement in the image at the time of exposure, from the camera or the subject. Landscapes and scenic panoramas are the choice subjects. Other subjects, such as moving cars, bikes, people, animals, even wind blowing through trees pose severe challenges to HDR photography. The most common problem is "ghosting." Ghosting is the result of camera or subject movement, if anything has moved from frame to frame, that movement will appear in the final HDR file as indistinct blurs or as "ghosts" stagering away from the main subject. Some dedicated software options offer ghosting corrections but only to a point.

There are of course a few downsides to HDR but I'll touch upon that in my next reply. Thanks for reading and looking.
 
Back
Top