Adobe Photoshop CS5 for Photographers - Martin Evening [152]
2.
In most instances it will be necessary to adapt the shape of the curve in the composite channel and individual color channels to achieve an optimum negative to positive conversion.
3.
In the bottom example I added a saturation boost to the image. Because of the extreme nature of such adjustments, it is better to work on an image that has been scanned in 16-bits per channel.
Locking down portions of the curve
As you go beyond adding one or two points to a curve, you need to be careful to keep the curve shape under control. Once you start adding further points to a curve, adjusting a point on one part of the curve may cause the curve shape to move, pivoting around the adjacent points. One solution is to sometimes lay down ‘locking’ points on the curve, as shown in Figure 5.39.
Figure 5.39 In this example I wanted to make the dark tones darker, but without affecting the mid to highlight tones so much. Here is what I did. I placed one curve point on the middle intersection point of the curve and another on the 75% intersection point. I then added a third curve point towards the toe of the curve and dragged downwards to darken the shadows and steepen the curve in the shadow to midtone areas. You will notice that because I had added the other two curve points first, adjusting the third curve point had very little effect on the upper portion of the curve.
Creating a dual contrast curve
Now let's look at adding even more curve points. There are times where you may want to manipulate the contrast in two sections of the curve at once. For this you may need to place three or more points on the curve such as in the Figure 5.40 example shown below. You are now entering dangerous territory, because if you are not careful you can end up either making some of the tones look solarized, or possibly flatten them, and in the process lose a significant amount of tonal detail.
Figure 5.40 In this example I wanted to boost the contrast in the shadows and the highlights separately. To do this, I applied the curve shape shown here where you will see that by using five curve points I was able to lock down the middle portion of the curve and independently steepen the shadow/midtone and highlight portions of the curve. Note that in order to prevent the curve from boosting the color saturation, I set the Curves adjustment blend mode to Luminosity.
Brightness and Contrast
For as long as Photoshop has been around, there has been an adjustment control called Brightness/Contrast. For years now many authors and experts have done their best to discourage people from using this adjustment control. This was because of the potential harm it could do to your pictures. Yet so many people continued to use it, oblivious to the fact that it was never intended for photographic use. However, since Photoshop CS3, the default behavior has been changed so that the Brightness/Contrast adjustment can roughly match the behavior of the Brightness and Contrast sliders found in Camera Raw (see Chapter 3), where the shadow and highlight endpoints are preserved and the image adjustments are constrained to operate between these two points. However, the old legacy behavior is still available as an extra option (Figure 5.41). The main reason for retaining the old style behavior is to maintain compatibility with legacy images where the old style Brightness/Contrast adjustment may have been recorded as part of a Photoshop action.
Figure 5.41 The Histogram panel can be used to demonstrate why the legacy Brightness/Contrast image adjustment setting is unsuited for photographic tonal corrections. The histogram on the left shows the normal histogram. If you use the Brightness slider to increase the Brightness, all you are doing is shifting all the tones lighter and as a consequence of this the highlight detail may become clipped. If you increase the Contrast, the tones are stretched in both directions and both the shadows and highlights