Adobe Photoshop CS5 for Photographers - Martin Evening [138]
WYSIWYG image editing
If you want true WYSIWYG editing (what you see is what you get) it is important that you follow the instructions laid out in the Chapter 2 on how to calibrate the display and configure the color settings. Do this and you will now be ready to start editing your photographs with confidence.
The image histogram
The histogram graphically represents the relative distribution of the various tones (referred to as Levels) that make up a digital photograph. For example, an 8-bit per channel grayscale image has a single channel and uses 256 shades of gray to describe all the levels of tone from black to white. Black has a levels value of 0 (zero), while white has a levels value of 255 and all the numbers in-between represent the different shades of gray going from black to white. The histogram is therefore like a bar graph with 256 increments, each representing how frequently a particular levels number (a specific gray value) occurs in the image. Figure 5.10 shows a typical histogram such as you'll see in the Histogram, Levels and Curves panels. This diagram also shows how the appearance of the graph relates to the tonal structure of a photographic image.
Now let's look at what that information can actually tell us. The histogram graphically shows the distribution of tones in a digital image. A low-key photograph (such as the one shown in Figure 5.10) will have most of the peaks on the left. Most importantly, it shows the positioning of the shadow and highlight points. When you apply a tonal correction using Levels or Curves, the histogram provides visual clues that help you judge where the brightest highlights and deepest shadows should be. The histogram also tells you something about the condition of the image you are editing. If there are peaks jammed up at either ends of the histogram, this suggests that either the highlights or the shadows have become clipped and that when the original photograph was captured or scanned it was effectively under- or overexposed. Unfortunately, if the levels are clipped at either end of the scale, you can't restore the detail that has been lost here. If there are gaps in the histogram at this stage, this most likely indicates a poor quality original capture or scan, or that the image had previously been heavily manipulated.
Figure 5.10 Here is an image histogram that represents the distribution of tones from the shadows to the highlights. Because this photograph mostly contains dark shades of gray, you will notice that the levels are predominantly located to the left end of the histogram. The height of each bar in the histogram indicates how frequently each levels value is represented in the image based on a 0–255 scale.
Figure 5.9.
The warning triangle in the Histogram panel indicates that you need to click on the Refresh button above to update it.
The Histogram panel
There has always been a histogram in the Levels dialog and there is also a Histogram panel, which when working in Photoshop gives you even more feedback. With the Histogram panel, you can continuously observe the effect your image editing has on the image levels and you can also check the histogram while making any type of image adjustment. The Histogram panel only provides an approximate representation of the image levels. So, to ensure that the Histogram panel is giving an accurate representation of the image levels, it is advisable to force Photoshop to update the histogram view, by clicking on the Refresh button at the top of the panel.
Throughout this book I will try to guide you as best I can to work as efficiently and as non-destructively as possible. Even so, anything we do to adjust the levels