By Joel and Hitomi Dames
Dynamic Range
I bought my last film camera, a Canon EOS 1N in mid-1990. We switched to digital when the first Canon Rebel came out in 2003. It was an exciting move from negative film to digital, but a clunky step-down at that time as far as dynamic range.
Dynamic range refers to tonal range, the difference between the brightest and darkest luminance you can capture. With film you might overexpose by 2 or 3 stops without a care. You had to be really shabby to blow out a scene, because film has so many more tones to work with between dark and light tones.
Histogram
The horizontal axis of the histogram (
see previous article) are tonal variations with 0 (total darkness) on the left side and 255 (whiteout) on the right. The vertical axis is the number of pixels in each tone.
The histogram shows tonal distribution of the millions of pixels captured in the image. For good exposure, the curve should be touching, but not scrunched against the right side. That way you capture as much exposure as possible in the shadows without blowing out the brightest tones.
Exposure Value ( EV )
Dynamic range is expressed as Exposure Value (EV). EV are tones from shadows to highlights expressed as a number. A scene in direct sunlight has an EV of 15. A Christmas tree of lights has an EV of 4 or 5.
The Nikon D7200 ( about $1200 ) has a landscape dynamic EV range of 14.6 stops. The top of the line Canon EOS 1DX ( about $5,000 ) has An EV landscape range of 11.8. But if a camera has a dynamic range of 8 EV stops, you will usually get good exposure, and most new cameras go beyond a range of 8 EV.
There are times, though, when the tones from shadow to highlight of a scene go beyond what you can capture in 8 stops. You must choose between exposing for shadow and blowing out highlight, or capturing highlight and letting shadows go black. A wedding dress in bright sunlight exemplifies the problem.
We can perceive the fine details of the white wedding dress, because we can perceive an EV range of about 24 luminance stops. A method of capturing that wedding dress in bright sunlight that comes close to what we perceive is with a high dynamic range ( HDR ) capture.
Creating HDR Images
HDR is simple and the results can be stunning. Take three exposures of the same image and fuse them together into a single image. Though some new cameras come with built in HDR, for creative control of the final image, many photographers use a program called Photomatix.
The camera with built in HDR takes three images, but we take as many as twelve. My wife, Hitomi, prefers a natural look. I sometimes want my images to pop. With HDR images, we experiment with both and usually end up somewhere between. We are the deciders, not the camera.
We shoot an annual apprentice graduation for the
Seattle Mechanical Contractors Association. In 2010, the ceremony took place at the Museum of Flight. When I saw the huge open space with airplanes dangling from the ceiling, I decided to take an HDR shot for the cover image. I wanted to get all the details in the shadow and not blow out the bright window light.
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| Low-dynamic range underexposed image |
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| Low-dynamic range overexposed image |
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| High dynamic range HDR image |
This was not really a good scene for HDR, though, because people were moving about and multiple exposures with people moving cause ghosting. As it turned out, though, I like ghosting in this image, since it gives an ambience of motion.
Bracketing Exposures
For an HDR image, you bracket exposures, overexposing to illuminate dark areas and under exposing to tone down overexposed highlights. You don't need to sacrifice shadow for highlight or highlight for shadow. Whether you rely on the camera or do it yourself, it is possible to get a good composite with just three exposures: overexposed, underexposed, and in-between exposure.
If you underexpose two stops and overexpose two stops, you extend the tonal range considerably. I always do that for an HDR, but sometimes I add two additional images, one underexposed four stops and another overexposed four stops. I end up with an expanded tonal range, and a balanced, if not eye-popping HDR image when I combine the five images (as opposed to just three) in Photomatix.
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