The following is an excerpt from The Adobe Photoshop Lightroom 5 Book: The Complete Guide for Photographers by Martin Evening.
Correcting an Overexposed Image
Adobe Photoshop Lightroom has the ability to reveal highlight detail that might otherwise be hidden. You can often recover seemingly lost highlight information by combining a negative Exposure adjustment with the use of the Highlights slider. Although Lightroom can recover the highlight detail on most images, it will have a limited effect on pixel-based images such as JPEGs, PNGs, or TIFFs. For best results, you can only use this technique when processing raw images. This is because Lightroom is able to use all of the luminosity information that’s contained in a raw file that is simply waiting for you to access it. In the accompanying example, I was able to recover one-and-a-half stops of overexposure, but in some cases it may be possible to recover as much as two stops.
NOTE: The ability to recover overexposed highlight detail is also dependent on the capture abilities of the sensor. Not all sensors are the same, and with some cameras you do have to be very careful not to overexpose.
It is often better to optimize the camera exposure to capture as much of the shadow detail as possible, but without overexposing to the point where you are unable to process important highlight information. I will often ignore the camera or light meter readings and deliberately overexpose at the time of capture in order to record the maximum amount of levels information and use the combination of negative Exposure and Highlights adjustments when processing the image in Lightroom.
How this works is that Lightroom features an internal technology called “highlight recovery,” which is designed to help recover luminance and color data in the highlight regions whenever the highlight pixels are partially clipped—in other words, when one or more of the red, green, and blue channels are partially clipped, but not all three channels are affected. The highlight recovery process initially looks for luminance detail in the non-missing channel or channels and uses this to build luminance detail in the clipped channel or channels. After that Lightroom also applies a darkening curve to the highlight region only, and in doing so brings out more detail in the highlight areas. But as I have just mentioned, this technology is designed to work with raw files, although JPEG images can sometimes benefit too (but not so much). Process 2012 has taken this further to provide improved highlight color rendering, which preserves the partial color relationships, as well as the luminance texture in the highlights. You should now find that highlight detail is rendered better (try reprocessing your sunset photos using Process 2012).
There is also less tendency for color detail to quickly fade to neutral gray and there is better preservation of the highlight detail. In Figure 1 you can see a direct comparison between working in Process 2010 and Process 2012 on an image where the highlights were burned out (and you can’t get much more burned out than shooting directly into the sun).
Figure 1 This shows a comparison between an extreme highlight recovery in Process 2010 (top) and Process 2012 (bottom).
If the highlights are completely blown out in the original image, you will never be able to recover all the detail completely, but using the latest Process 2012 I think you’ll be pleasantly surprised at the difference when reprocessing some of your older Process 2003/2010 shots, especially photographs like the one shown in Figure 1.
The overexposed photograph below was initially processed using just the default Basic panel settings in the Develop module. The histogram shows severe clipping in the highlights, and you can see how there is not much detail in the sky. A histogram like this can appear disconcerting until you realize that there is more information contained in the image than appears at first sight.
The main treatment for an overexposed photo is to apply a combination of negative Exposure, Highlights, and Whites adjustments, though I mainly used the Exposure slider to achieve the desired darkening.
Correcting an underexposed image
Underexposed images represent a bigger problem because there will be fewer levels available to manipulate, particularly in the shadows. However, the Basic panel controls in Lightroom can be used to brighten an image and lift out the shadow detail.
The way you need to approach this is to mainly drag the Exposure slider to the right until the image begins to have about the right brightness. As you do so, don’t worry too much about the shadows just yet, because the next step will be to adjust the Shadows slider by dragging this to the right to bring out more detail in the shadow regions of the image. Beyond that, it’s all about fine-tuning the image. In the example shown here, I needed to reduce the Highlights to preserve tonal detail in the highlight areas and I also needed to reduce the Blacks in order to compensate for a strong Shadows adjustment and maintain a decent amount of contrast in the darker areas. You will also want to watch out for deteriorating shadow detail. As I mentioned above, brightening up a dark photo can reveal problems in the shadows such as tone banding and noise.
As with the highlight recovery method earlier, underexposure corrections should mainly be
done by adjusting the Exposure slider first in order to achieve the right level of brightness.
In this example, I dragged the Exposure slider to the right, which lightened the image considerably. But because I was lightening for the midpoint, this adjustment also over-brightened the highlight areas in the clouds. To compensate for this, I applied a negative Highlights adjustment. I also added a positive Shadows adjustment to lighten the dark areas of the photo. Finally, I applied a negative Blacks adjustment to ensure the shadows were clipped correctly and also to add more contrast in the shadow region. The end result is a photo that is quite usable, considering how dark it was before. However, lightening such a dark original will have also amplified the noise—this may be especially noticeable in the shadow areas.
Excerpted from The Adobe Photoshop Lightroom 5 Book: The Complete Guide for Photographers by Martin Evening Copyright © 2013. Used with permission of Pearson Education, Inc. and Adobe Press. All rights reserved.