Friday, April 2, 2010

Overexposing Raw

Most photographers try to make the perfect exposure every time.
Text & Photography By William Sawalich Published in Shooting
Overexposing Raw
A “normal” exposure’s histogram will show an even distribution.

Most photographers try to make the perfect exposure every time. But there’s a school of thought that says photographers who capture RAW files would do better to slightly overexpose. This theory, sometimes called “exposing to the right” because it places the peaks of a histogram right of center where brighter tones are represented, is based on the goal of improving the signal-to-noise ratio. With less noise and more image-forming information captured in the brighter tones, subtle overexposure of RAW images can yield more detail and lower noise in finished photographs.

Overexposed images will result in larger image files. More light captured by the sensor translates to more image data. Compare the file sizes of the two exposures above.

For years, film photographers were taught to increase density by overexposing negatives and underexposing transparencies. A denser negative or subtly underexposed chrome provided more image-forming information. With digital files, though, the image is formed by an electrical charge that’s created by light. The brighter the pixel, the more light it has captured—and the more image-forming information contained therein.

Don’t believe me? Photograph the same scene in RAW twice, once overexposed and once under. The overexposed image file will be significantly larger than the darker image because it contains more data. In the example shown, the underexposed scene produced a 13.3 MB RAW file, while the overexposed version was almost 16 MB. That’s five percent more data for better detail and an improved signal-to-noise ratio.

To appropriately overexpose RAW files, though, less is usually more. You don’t want to overexpose too much or you’ll blow everything out. When “exposing to the right,” the idea is to move the peaks on the histogram toward the right side of the chart without clipping. Clipping means blowing out highlights by overloading the pixel with a charge and eliminating every last bit of detail. Once a highlight is gone, no amount of postproduction recovery can bring it back.

The top histogram illustrates underexposure, which may result in a lack of details in the shadow areas. The bottom histogram is a recommended overexposure. Note that the distribution is heavy to the right—be careful of going too far or you’ll irretrievably lose detail in the highlights.

The trick is to overexpose appropriately. Try a half stop, a full stop or even more overexposure in a low-contrast situation of normal brightness. You usually can overexpose more without clipping if you’re photographing a low-key scene. Go too far and you risk blown-out highlights, especially on bright sunny days or in high-key scenes. The best approach is to keep an eye on the histogram and check for “zebras” on the camera’s LCD. These blinking highlights indicate too much overexposure. Limit them to select areas where you want pure white without detail and you’ll do just fine.

After making a slightly overexposed RAW file, your job is only halfway done. The next step is to use your RAW editor’s adjustment sliders to ease the exposure back to a normal level, paying special attention to highlight details. At this point, you’ll notice the major benefit of subtle overexposure—lower noise and higher detail, particularly evident in the shadows and when shooting at high ISOs.

Take a look at the three sample landscape photos after adjustment. Notice how there’s significantly more noise in the underexposed image. Along with that noise came contrast and a general lack of detail when the underexposed RAW file was “normalized” in post. There’s a bit of noise in the shadows of the normally exposed image, too—that’s to be expected at a higher ISO. But most interesting is the overexposed shot. Not only is there more detail in the shadows, but that detail also remains in the highlights (unlike the underexposed image), and there’s a whole lot less noise throughout.

Noise Detail (Normal Exposure)

Noise Detail (Overexposure)

Noise Detail (Underexposure)

The benefits of lower noise and increased detail with overexposure mean that you have effectively lowered your shooting ISO and improved the dynamic range by maintaining shadow details that otherwise would have been lost. As long as you haven’t overexposed so much as to clip the histogram and blow out highlights, the image can be recovered easily to increase detail and decrease noise. In fact, it’s good practice for all digital exposures to start with the highlights. Avoid clipping at all costs, but maximize brightness to maximize detail and make the most of the RAW advantage.
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