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the number of levels that are available to record an exposure (see Figure 3.36 ). Let us suppose that the optimum exposure for a particular photograph at a given shutter speed is f16. This exposure makes full use of the chip sensor's dynamic range and consequently there is the potential to record up to 4096 levels of information. If one were then to halve the exposure to f22, you would only have the ability to record up to 2048 levels per channel. It would still be possible to lighten the image in Camera Raw or Photoshop to create an image that appeared to have similar contrast and brightness. But (and it's a big but) that one stop exposure difference has immediately lost half the number of levels that could potentially be captured by using a one stop brighter exposure. The image is now effectively using only 11 bits of data per channel instead of 12. This is true of digital scanners too. Perhaps you may have already observed how difficult it can be to rescue detail from the very darkest shadows, and how these can end up looking very posterized. Also, have you ever noticed how much easier it is to rescue highlight detail compared with shadow detail when using the Shadows/Highlights adjustment? This is because far fewer levels are available to define the information recorded in the darkest areas of the picture and these levels are easily stretched further apart as you try to lighten the shadows. This is why posterization is always much more noticeable in the shadows (see Figure 3.35 ). It also explains why it is important to target your digital exposures as carefully as possible so that you capture the brightest exposures possible, but without the risk of blowing out the highlight detail.

Figure 3.36 If you could inspect a raw capture image in its native, linear gamma state, it would look something like the image shown top left. Notice that the picture is very dark, it is lacking in contrast and the levels (representing the tonal information) in the histogram appear to be mostly bunched up to the left. During the raw conversion process, a gamma curve correction is applied when converting the linear data so that the processed image matches the way we are used to viewing the relative brightness in a scene. The picture top right shows the same image after a basic raw conversion. As a consequence of this, the more brightly exposed areas will preserve the most tonal information and the shadow areas will end up with fewer levels. A typical CCD sensor can capture up to 4096 levels of tonal information. Half these levels are recorded in the brightest stop exposure range and the recorded levels are effectively halved with every stop decrease in exposure. The digital camera exposure is therefore quite critical. Ideally, you want the exposure to be as bright as possible so that you make full use of the Levels histogram, but at the same time be careful to make sure the highlights don't get clipped.

Figure 3.35 This shows the difference the exposure can make in retaining shadow information. The darker the exposure, the fewer discreet levels the CCD chip can capture and this can result in poorly recorded shadow detail.

Camera histograms

As I have mentioned already in this book, the histogram that appears on a compact camera or digital SLR screen is unreliable for anything other than JPEG capture. This is because the histogram you see there is usually based on the camera-processed JPEG and is not representative of the true raw capture. The only way to check the histogram for a raw capture file is to open the image via a raw processing program such as Camera Raw or Lightroom.

Brightness

There is sometimes some confusion between when to use Brightness and when to use Exposure to adjust an image. To the newcomer they both appear to be doing the same thing, but there is in fact an important difference between the way these two adjustments work.

The Exposure slider is essentially a white clipping point tool that is used to set the highlight clipping point. As you move the slider left or right the image appears to get