Exposing For Raw

When shooting in raw mode, it's worth considering a change in your exposure strategy.

Most digital photographers are used to underexposing to protect their highlights. Sure, underexposing can lead to shadow tones that are completely black, but an underexposed shadow is much less visible than an overexposed highlight. What's more, because Aperture can rebuild highlights that have been slightly overexposed, you have a bit of a safety net when shooting in raw mode.

However, because of the linear nature of digital image sensors, if you take a different approach to your exposures, you'll produce images that contain much more robust data, which can withstand more edits. You'll also find that you can produce images that are less prone to noise.

Linear Data Revisited

As discussed earlier, the image sensor in your digital camera has a linear response to light. When the brightness in your scene doubles, the camera registers twice as much light.

Camera sensors begin recording an image by measuring the brightest values in your scene. This brightest stop takes up fully half of the dynamic range of your sensor. Thus, if your sensor is capable of capturing 4,096 levels of light, 2,048 of those levels will be consumed in the recording of just the brightest stop in your scene.

The next brightest stop will use half of those remaining 2,048 levels. The next brightest will use half of what's left over from that, and so on for about 8 to 10 stops worth of light (Figure 6.16).

Figure 6.16. Most of the data that your camera captures goes to recording the brightest half of the image. Half of the remaining data goes to recording the next stop, and so on. This means that your camera records substantially more information for the bright areas of your image than for the dark areas.

Thus, when you get to the darkest tones in your scene, your camera will be representing them with only a few shadesin some cases, very few shades. Consequently, if you expose to preserve as much highlight detail as possible, then you'll be capturing far more levels than if you expose for shadows.

But, you may be thinking, what if the scene I'm envisioning in my final output is a dark scene? That's fineyou should still expose to capture as many of the bright tones as you can, because you'll get far more information, and you can always push those tones down into the shadow areas to create a darker image. What's more, you'll end up with shadow areas that are much less noisy than if you try to expose to capture shadow tones. Because your camera captures so few tones in the shadow range, the signal-to-noise ratio in those tones is very bad. By capturing high-quality highlights and then darkening them, you can achieve much less noisy shadow areas.

So when shooting, you want the data in your histogram biased more toward the right edge than toward the left to guarantee that you're capturing as much data as possible in the area where your camera records the most information.

If you're worried about overexposure, remember: you have a little bit of a safety net in the form of Aperture's highlight recovery features. To play it safe while you learn this technique, though, bracket your shots and shoot at least one exposure using your normal strategy.