High Dynamic Range (HDR) Digital Photography Expands One's Creative Range

High dynamic range (HDR) photography captivates viewers through its bedazzling attention to details and its stunning array of color. Rich and beautiful images created using HDR techniques directly responds to the age old problem of not being able to record what the eye can see. HDR overcomes this limitation and more.
From the earliest days of photography, the days when photographers created glass negatives using a mixture of silver nitrate and egg whites, photographers complained about being unable to record a scene on film as the eye sees that scene. Many photographers learned to master these limitations, think about Ansel Adams' Zone System for black and white photography, learning to manipulate exposure with filters, film sensitivity, developing processes, and much darkroom gymnastics. The result was, and remains, stunning imagery. For amateurs and serious hobbyists, however, found themselves fighting against murky shadow detail and washed out skies. The struggle was real even for the seasoned professional.
The problem originates in the real world. The dynamic range of what the eye can see across the visible spectrum is enormous. Our eyes evolved to see the entire range from deep shadow to bright highlight and everything in between. Capturing that range on film or using a digital sensor presents one with an efficiency problem. Film, digital sensors and the techniques for printing not to mention computer monitors, are simply not up to the task of capturing detail across a wide dynamic range. HDR photography changed all that forever. Photographers using HDR techniques are now able to capture light across the widest dynamic range with relative ease.
What is this dynamic range I keep referring to? In its simplest terms the dynamic range refers to the variation in luminance from the brightest to the darkest light values in any given exposure and high dynamic range photography refers to the ability to capture light across all or most of the dynamic range of the scene being photographed. HDR is accomplished by exposing from 3 to 7 bracketed exposures across a range of f-stops and/or shutter speeds and then recombining those exposures into a single merged image which now reflects exposures made for highlights, midtones as well as shadows.
Every bracketed image contributing to a finished HDR image inputs important information about the image being created. Underexposed images contribute to highlight detail while overexposed images provide information about the shadows. When combined, the bracketed exposures produce an hdr file that contains all of the information necessary to produce the hdr image. A second step, often referred to as tone mapping, converts the hdr file into a usable image that may then be saved as a TIFF or JPEG file. The tone mapped image is the one that is most useful when we print an HDR image. The tone mapped image displays the full range of color and detail in both the shadow areas and the highlight areas while holding the midtones true and rich.
HDR doesn't just happen. This should come as no surprise. No photography, at least not good photography, ever just happens and HDR is certainly no different. Long before the first picture is taken one must have a good feel for the technical aspects of the equipment being used. Knowing how to successfully bracket, how many exposures to make and what intervals are appropriate for a bracketed series is important. Knowing how to manage images on your computer is a boon to speeding your workflow as you process your images. Paying attention to the details of HDR goes a long way to helping you create rich HDR images.
Is HDR perfect? Heck no! It is appropriate for some but not all conditions, it is not appropriate for all subjects. In fact, most subjects that display a large degree of motion are generally not candidates for HDR photography. But when the conditions and subjects are right, HDR solves many dynamic range problems.

Dynamic Range in the World of Photography

Look at something other than your computer screen for a moment and take note of the world around you. You should notice bright highlights and dark shadow areas without much of a problem. Film and digital sensors do not see as well as you do. The shadows and highlights may be quantified as numbers that represent the luminance of any scene. The standard measure of luminance is expressed as candelas per square meter or cd/m2. The luminance of the sun is expressed as a ratio of 1,000,000,000:1 or 1 billion candelas to 1 square meter. Other common expressions of luminance are:

Star light = 0.001:1



Moon light = 0.1:1



Indoor light = 50:1



Sunny sky = 100,000:1

So what does all this mean to me as a photographer? Well, if an average sunny day has a luminance of 100,000:1 this means that the brightest area of a scene is 100,000 times brighter than the darkest area. Of course, all outdoor scenes are not fixed at 100,000:1. Haze, clouds, early morning and late afternoon sun all impact the dynamic range of an image. Shooting at noon is far different than shooting during the golden hours. I generally avoid shooting outdoors between around 10:00 AM and 2:00 PM because even with HDR as a tool, the harshness of the dynamic range does not make for pleasing images.

For our purposes here think of stops as exposure values or EV's. An EV is an integer, a whole number, corresponding to the scene luminance. Under this formula an EV=0 when the correct exposure is 1 second at f/1.0. An EV increment of 1 is the equivalent of 1 stop so that an increase in EV by 1 halves the amount of luminance while an increment of 1 stop down doubles the luminance or available light. The human eye has a dynamic range of 100,000:1 which, in turn, is equivalent to a range of 20EV. Below I show some common mechanical values for a number of capture devices as well as some display medium.

Negative film: dr=1500:1 or 10.5EV



Computer monitor: dr=500:1 or 9.0EV



D-SLR: dr=300:1 or 7.0EV



Compact digital camera: dr=100:1 or 6.6EV



High quality glossy print:  dr=200:1 or 7.6EV



High quality matte print:  dr=50:1 or  5.6EV

So there you have it, the crux of the problem. If the scene outside has a dynamic range of say 50,000:1 and the sensor on your very high quality, professional grade D-SLR only has a sensor capable of capturing a dynamic range of 300:1 how does one capture and display images that have a higher exposure value than the technology is capable of capturing or displaying?

Let's look at how images are captured in your camera as a lead into answering our primary question of how to capture images that are mechanically uncapturable. Digital sensors replace film in the modern world so I will limit this discussion to digital photography. Many D-SLR cameras and some others support a RAW format for image capture. Canon's CRW and CR2 files or Nikon's NEF files are examples of proprietary RAW captures. A single RAW file records around 10EV. Pretty good, eh, but still not quite good enough to get everything that's out there. The advantage of the RAW file, however, is that it captures a continuum of stops from under to over exposed in a single file which can prove advantageous later on. Some users, because of the extended EV have called the RAW file a medium dynamic range image.

Cameras also store images as JPEG files. The sensor interpolates the color and intensity and sends the exposure through a series of in-camera steps that address white balance, saturation, sharpness, contrast and so on. JPEG compression, the final step in the process, is performed and the file is stored as a JPEG file. The JPEG file contains only 256 levels of intensity and captures only about 8EV. It is a low dynamic range image. For most studio work the JPEG file is the file of choice for me. It cuts down on my workflow and processing time and for portraiture where I am in full control of the light and the dynamic range of that light, I find no need to shoot RAW. Landscapes, on the other hand, I only shoot in RAW.

The two standard formats for saving images after the RAW conversion takes place are TIFF and JPEG. While JPEG files can be produced in-camera from the RAW exposure using proprietary software in the camera, TIFF files are created during processing of RAW files in processing software such as Photoshop or Lightroom. The JPEG file contains luminance values ranging from 0-255 (a total of 256) while the TIFF file stores values from 0-65,535. It is clear that the TIFF file stores a far wider luminance range than does the JPEG file.

Still, even the TIFF file cannot capture the full dynamic range of a beautiful landscape. In order to produce a high dynamic range image one must look elsewhere. Two formats for storage are available, the RadianceRGBE (.hdr) and OpenEXR (.exr) generally produced in software outside of Photoshop or Lightroom, I use Photomatix PRO for my HDR conversions from RAW files and generally save the RadianceRGBE format. The RadianceRGBE format is a 32-bit format while the OpenEXR format starts out as 48-bits but is reduced in processing to 32-bits. Both formats are lossless (saving and reopening does not degrade the files). The RadianceRGBE format represents 76 orders of magnitude in dynamic range, remember that the human eye has a mere 20EV range, far more than one might ever need. The drawback, although I have never been able to really see the difference, is that the RadianceRGBE format is slightly less accurate than the OpenEXR format.

Once the hdr conversion takes place, that is you now have a.hdr or.exr file there is one more step required. In its native state the hdr files are unusable. One must tone map the hdr file in order to make it useful. Tone mapping is a scaling back process taking the 32-bit hdr file and reducing the 32-bit floating point values present in the HDR file and producing a 16-bit TIFF or 8-bit JPEG file containing fixed integer values. Now you have an image that mirrors the high dynamic range of the landscape you are photographing. While not perfect, the HDR process is a quite good answer to the problem of capturing the uncapturable.