Adobe Photoshop CS5 for Photographers - Martin Evening [142]
The RGB edit space and color gamut
One of the first things you need to do when you configure Photoshop is to choose an appropriate RGB edit space from the RGB Working Spaces menu in the Edit Color Settings dialog (Figure 5.12).
Figure 5.12 The Color Settings dialog.
For photo editing work, the choice really boils down to Adobe RGB or ProPhoto RGB. The best way to illustrate the differences between these two RGB color spaces is to consider how colors captured by a camera or scanner are best preserved when they are converted to print. Figure 5.13 shows (on the left) top and side views of a 3D plot for the color gamut of a digital camera, seen relative to a wire frame of the Adobe RGB working space. Next to this you can see top and side views of a glossy inkjet print space relative to Adobe RGB. You will notice here how the Adobe RGB edit space clips both the input and output color spaces. This can be considered disadvantageous because all these potential colors are clipped as soon as you convert the capture data to Adobe RGB. Meanwhile, Figure 5.14 offers a direct comparison showing you what happens when you select the ProPhoto RGB space. The ProPhoto RGB color gamut is so large it barely clips the input color space at all and is certainly big enough to preserve all the other colors through to the print output stage. In my view, ProPhoto RGB is the best space to use if you really want to preserve all the color detail that was captured in the original photo and see those colors preserved through to print.
Figure 5.13 This diagram shows, on the left, a top and side view of the gamut of a digital camera source space plotted as a solid shape within a wire frame shape representing the color gamut of the Adobe RGB edit space. On the right is a top and side view of the gamut of a glossy inkjet printer color space plotted as a solid shape within a wire frame of the same Adobe RGB space.
Figure 5.14 This diagram shows, on the left, a top and side view of the gamut of a digital camera source space plotted as a solid shape within a wire frame shape representing the color gamut of the ProPhoto RGB edit space. On the right is a top and side view of the gamut of a glossy inkjet printer color space plotted as a solid shape within a wire frame of the same ProPhoto RGB space.
Is ProPhoto RGB too big a space?
In the past there have been concerns that the ProPhoto RGB space is so huge that the large gaps between one level's data point and the next could lead to posterization. This might have been a valid argument when images were mainly edited in 8-bits per channel throughout. In practice, you can edit a ProPhoto RGB image in 16-bits or 8-bits per channel mode, but 16-bits is safer. Plus these days you can use Camera Raw to optimize an image prior to outputting as a ProPhoto RGB pixel image.
The other choice I should mention is the sRGB color space, but this is only really suited for Web output work (or when sending pictures to clients via email).
Direct image adjustments
Most Photoshop image adjustments can be applied in one of two ways. There is the traditional, direct adjustment method where image adjustments can be accessed via the Image Adjustments menu and applied to the whole image (or image layer) directly. Figure 5.15 shows an example of how one might apply a basic Levels adjustment. Here, I had an image open with a Background layer, then went to the Image Adjustments menu and selected ‘Levels…’ This opened the Levels dialog where I was able to apply a permanent tone adjustment to the photograph.
Figure 5.15 This shows the basic workflow for applying a normal image adjustment. Go to the Image Adjustments menu and choose an adjustment type. In this example I selected ‘Levels…’, adjusted the settings and clicked OK to permanently apply the adjustment to the image.
Direct image adjustments are appropriate for those times where you don't need the editability that adjustment layers are able