Tone Editing Controls in Adobe Photoshop Lightroom 4
The tone adjustment controls are meant to be applied in the order they appear listed in the Basic panel (Figure 4.26). When using Process 2012, the adjustment outcome with each slider is to some extent linked to the image content. Here then is a summary of what the Process 2012 sliders do.
The Exposure slider is both a midtone brightness and highlight clipping adjustment. This means that when evaluating an image you use the Exposure slider to adjust the image to get it to look the right brightness. If you set the Exposure too dark you won t be exploiting the full tonal range into the highlight areas. As you lighten with the Exposure slider the image will become progressively lighter and as you approach the point where the highlights might potentially become clipped, the brightening adjustment smoothly ramps off toward the highlight end, which helps preserve detail in the highlight areas. As you push the Exposure slider further you will then start to clip the brightest highlights. Mainly, you want to use the Exposure to get the image brightness looking right. From there on, no matter what you do with the other tone sliders, the midpoint brightness value won t shift too much until you make a further Exposure slider edit.
The Exposure slider s response correlates quite well with the way film behaves, but is also dependent on the image content. Previously, with Process 2003/2010, as you increased the Exposure the highlights would at some point hard clip. Also, as you increased the Exposure slider further, there was a tendency for color shifts to occur in the highlights as one or more color channels began to clip. With Process 2012, as you increase Exposure there is more of a soft clipping of the highlights as the highlight clipping threshold point is reached. Additional increases in Exposure behave more like a Process 2010 Brightness adjustment in that the highlights roll off smoothly instead of being clipped. As you further increase Exposure you will of course see more and more pixels mapping to pure white, but overall, such Exposure adjustments should result in smoother highlights and reduced color shifts. You should also find that it provides you with about an extra stop of exposure latitude compared to editing with the Process 2003/2010 Exposure slider.
If you hold down the key as you drag the Exposure slider, you ll see a Threshold mode view which highlights any highlight clipping. This may be seen as a useful guide to where clipping may be taking place, but I would not recommend you be so hung up about highlight clipping when using the Process 2012 Exposure slider compared to when making Process 2003/2010 adjustments. When using Process 2012 you need to judge the image brightness visually and reserve using the key Threshold view analysis when adjusting the Highlights and/or Whites sliders.
Understanding camera exposures
A typical CCD or CMOS sensor in a digital camera is capable of recording over 4,000 levels of information. If you are shooting in raw mode, the ability to record all these levels very much depends on a careful choice of exposure. The ideal camera exposure should be bright enough to record all the tonal information without clipping important highlight detail. This is because half the levels information is recorded in the brightest stop range. As shown in Figure 4.27, for each stop decrease in exposure, the number of levels that can be recorded are potentially halved. The upshot of this is that you do not want to deliberately underexpose an image, unless, that is, to do otherwise would result in the loss of important highlight detail. Deliberate underexposure can have a dramatic impact on the deep shadow detail, since relatively fewer levels are left to record the shadow information. Figure 4.28 shows how you can easily lose detail in the shadow areas due to an underexposure at the capture stage.
Figure 4.27 If you don t optimize the camera exposure, you may be missing the opportunity to record a greater number of levels via the sensor. The top diagram shows how a correctly optimized exposure makes maximum use of the sensor s ability to record the fullest amount of levels information possible. In the lower diagram you can see how recording the exposure just one stop darker than the ideal exposure results in only half as many levels being recorded by the sensor.
Figure 4.28 This image is divided diagonally. The top section shows the enhanced shadow detail using an optimum camera exposure setting, and the bottom section shows the same scene captured at minus two stops camera exposure and then processed to match the luminance of the normal exposure. Notice that there is more noise and less tonal information in the underexposed version.
As I mentioned earlier, if you are shooting raw, it is unwise to place too much emphasis on the camera histogram, since what you see here is based on a JPEG capture view. It is best to either trust the exposure system in the camera to get it right or rely on the histogram in Lightroom. In practice, I do sometimes check the histogram as I am shooting, just to make sure that at the very least I am not underexposing, according to what the camera shows me. But I won t be particularly worried If the camera histogram shows a few signs of highlight clipping.