Dynamic Range

The pictures shown in Figure 11.4 were taken within a minute of each other from a roof on a winter morning. Anyone who has ever tried to photograph a sunrise or sunset with a digital camera should immediately recognize the problem at hand. With a standard exposure, the sky comes in beautifully, but foreground houses are nearly black. Using longer exposures you can bring the houses up, but by the time they are looking good the sky is completely blown out.

Figure 11.4. Different exposures of the same camera view produce widely varying results.

The limiting factor here is the digital camera's small dynamic range, which is the difference between the brightest and darkest things that can be captured in the same image. An outdoor scene has a wide array of brightnesses, but any device will be able to read only a slice of them. You can change exposure to capture different ranges, but the size of the slice is fixed.

Our eyes have a much larger dynamic range and our brains have a wide array of perceptual tricks, so in real life the houses and sky are both seen easily. But even eyes have limits, such as when you try to see someone behind a bright spotlight or use a laptop computer in the sun. The spotlight has not made the person behind any darker, but when eyes adjust to bright lights (as they must to avoid injury), dark things fall out of range and simply appear black.

White on a monitor just isn't very bright, which is why our studios are in dim rooms with the blinds pulled down. When you try to represent the bright sky on a dim monitor, everything else in the image has to scale down in proportion. Even if a digital camera could capture extra dynamic range, you still couldn't display it on a monitor. And how would that extra range be stored in an image?

A standard 8-bit computer image uses values 0 to 255 to represent RGB pixels. If you could record a value above 255say 285 or 310that would represent a pixel beyond the monitor's dynamic range, brighter than white or overbright. Because 8-bit pixels can't actually go above 255, overbright information is stored as floating point decimals where 0.0 is black and 1.0 is white. Because floating point numbers are virtually unbounded, 0.75, 7.5, or 750.0 are all acceptable values, even though everything above 1.0 will clip to white on the monitor (Figure 11.5).

Figure 11.5. 8-bit and 16-bit pixels stop at white, while floating point can go beyond. Floating point also extends below absolute black, 0.0, values that are theoretical and not part of the world you see (unless you find yourself near a black hole in space).

In recent years, techniques have emerged for taking a series of exposures (such as the sunrise shots) and creating high dynamic range (HDR) imagesfloating point files that contain all light information from a scene (Figure 11.6). The best-known paper on the subject was published by Malik and Debevec at SIGGRAPH '97 (go to www.debevec.org for more details). In successive exposures, values that remain within range can be compared to describe how the camera is responding to different levels of light. That information allows a computer to connect bright areas in the scene to the darker ones and calculate accurate floating point pixel values that combine detail from each exposure.

Figure 11.6. Consider the floating point pixel values for this HDR image.

But with all the excitement surrounding HDR imaging, many forget that for decades there has been another medium available for capturing dynamic range far beyond what a computer monitor can display.

That medium is film.