High Dynamic Range (HDR), what is it, why should I know about it, and what can it do to broaden my photography? Three simple questions so often raised by those new to digital photography that are often difficult to answer. In this post I will try to give a straight forward overview of HDR that will allow you to experiment with the techniques to determine if HDR has a place in your photographic armamentarium.
First, what is dynamic range? Dynamic range defines the breadth of luminosities that can be visualized. Luminosity is the term used to define the brightness of an object. Luminosity along with hue (color) and saturation (intensity of color) determine how we see an object or the image of an object. Our eyes have an incredible ability to detect light simultaneously at low and high luminosity. This range is typically quantified in exposure values (EV) or more often as f-stops. One EV or one stop is the equivalent of doubling the about of light. The dynamic range of our eyes is about 18-24 stops – a huge range of light intensity from dark shadows to very bright highlights. Cameras don't do so well. A single image captured with a film camera may retain image detail over about 8 stops; a digital image captured with a very high-end dSLR camera may retain detail over a range of about 10-12 stops. In other words, our very best cameras cannot process an image with variations in tonality or brightness as well as our eyes.
This was a big problem in the days of film when painstaking dodging and burning were done in the darkroom but with the advent of digital imaging and sophisticated processing software we are able to produce HDR images with a tonal resolution nearly equal to our eyes. The theory of how this is done is simple. By capturing multiple images at different exposures, the processing software can use the low luminosity data from over-exposed images and high luminosity from under-exposed images and combine and process the data to give a greatly expanded dynamic range. In other word HDR gives you the ability to extract more image detail from both the bright and dark areas of a scene giving the image a feel closer to what you visualized with your eyes. However, overdoing HDR processing can transform a good image into a quirky, over-done, cartoon-ish image that has too much contrast and is too highly saturated. HDR can create a surrealistic feel to an image that may be desirable or not.
What subjects make the best HRD images? First, static subjects are usually much more appropriate for HDR processing than moving subjects. Subjects with texture and a wide range of tonality (variations between bright and dark) are often best for HDR. Often subjects with bright and highly saturated colors can be dramatic in HDR. People or wildlife that are stationary may be imaged using HDR processing but moving people or objects create problems with the processing and often do not work well.
You can imagine that the way to expand dynamic range of an image is to capture the bright and dark areas of a scene by intentionally over and under exposing the subject and that is exactly what we do. The shooting technique is to capture three or more identical images at different exposures. Always shoot in RAW format because you heed a large bit-depth to process the final image and JPEGs will always disappoint you when shooting HDR. To be certain that the images are identical it is desirable to use a tripod and capture the multiple images as rapidly as possible using the high-speed burst exposure mode of your dSLR. Most dSLRs allow you to use a mode called auto-bracket exposure – in other words, the camera rapidly captures 3 or more images at the exposure you have set and at an exposure that is intentionally over and under exposing the image. The exact way you do this is important. Your camera should be on a tripod and the mode set to aperture priority (Av on Canon and A on Nikon.) The reason for using aperture priority is that you want to change the exposure by changing shutter speed not aperture. If aperture were to change, you would create images with different depths-of-field and so focus/sharpness would be different between the highlights and the shadows. You can see why you want a static, non-moving subject because the variation of shutter speed and the timing of the multiple exposures would cause variation in the sharpness and position of the subject. This produces an artifact in the processed image called ghosting (more later.) Finally, you want your exposure bracketing to be at least 3-6 total stops. That is, if you are shooting 3 bracketed exposures, the shutter speeds should be a minimum of one to two stops higher and lower that the ideal metered shutter speed resulting in images that are significantly over and under exposed. I typically set my HDR bracketing on my 7D at 1 1/3 stops for three shots in a moderately well lighted scene. If there is large variation between the shadows and highlights I will increase the bracketing to 1 2/3 or 2 stops. The 7D like many dSLRs will only shoot 3 bracketed shots. On my 5DIII I set the bracketing at one stop and shoot 5 to seven bracketed shots. I generally shoot two sets of images for each composition and I try to remember to shoot each set in both horizontal and vertical formats.
Now that you have captured the images it is time to look at HDR processing. Some of today's newest cameras allow HDR processing in the camera. That said, most photographers want the control of post-production software to refine and tune their images to meet their exact needs. There are many dedicated HDR programs that work independently or with Adobe Photoshop to create the processed image. Each program has pros and cons beyond the scope of this introduction. The good news is that most of these programs can be down-loaded for a free trial period so you can choose the one with the features and results that you want. The two most popular stand-alone programs seem to be Photomatrix by HDRSoft and HDR Efex Pro by Nik Software. Both of these also work as plug-ins with Adobe Lightroom and Photoshop. There are also a growing number of free and shareware programs and, of course, Photoshop itself does HDR processing and this was my personal choice.
Each HDR program collects, aligns, and processes the 3-9 individual, bracketed images you have shot and down-loaded. The 12 or 14-bit images are converted to a huge 32-bit image for processing so you can imagine that you need some pretty sophisticated processing power in your computer. The images ate 'tone-mapped' to select the highlights and shadows that will have the most detail for the final image. Once the mapping is done you have the ability to select how you want to process the final image in terms of contrast, saturation, exposure, and the 'strength' of the HDR effect. All of the programs have a variety of presets to help you get close to your desired effect. You also have the ability to eliminate ghosting that may be present if there was movement in the scene during capture or incomplete alignment due to camera movement. Lastly, the image will be outputted for viewing as a very large PSD or TIFF file or as a compressed JPEG.
The results of HDR processing may surprise you, please you, or enrage you. You will have a new image that is high in contrast, color saturation, and details beyond what you could imagine. Done tastefully (dare I say properly) HDR images are a wonderful addition to your photographic repertoire but, overdone, they are less than pleasing. In all cases, HDR processing will give you new insights to your photography so you should give it a try.