Adobe Photoshop CS5 for Photographers - Martin Evening [286]
Custom RGB and workspace gamma
Expert users may wish to use an alternative custom RGB workspace in place of one of the listed RGB spaces. If you know what you are doing and wish to create a customized RGB color space, you can go to the Custom… option in the pop-up menu and enter the information for the White Point, Gamma and color primaries coordinates (Figure 12.33). My advice is to leave these expert settings well alone. Avoid falling into the trap of thinking that the RGB workspace gamma should be the same as the monitor display gamma setting. The RGB workspace is not a display space.
Figure 12.33 You can use the Custom RGB dialog to create custom RGB workspaces. The settings shown here have been named ‘Bruce RGB’, after Bruce Fraser who once devised this color space as a suggested prepress RGB space for Photoshop.
Adobe RGB is considered a good choice as an RGB workspace because its 2.2 gamma provides a more balanced, even distribution of tones between the shadows and highlights, while others, like myself prefer the 1.8 gamma ProPhoto RGB space for its wide color gamut. Remember, you do not actually ‘see’ Adobe RGB or ProPhoto RGB and the RGB workspace gamma has no impact on how the colors are displayed on the screen (so long as Photoshop ICC color management is switched on). In any case, these advanced custom color space settings are safely tucked away in Photoshop and you are less likely to be confused by any apparent discrepancies between the display gamma and the RGB workspace gamma.
RGB to CMYK
Digital scans and captures all originate in RGB but professional images are nearly always reproduced in CMYK. Since the conversion from RGB to CMYK has to happen at some stage, the question is: at what point should this take place and who should be responsible for the conversion? If you have decided to take on this responsibility yourself then you need to understand more about the CMYK settings. When it comes to four-color print reproduction, it is important to know as much as possible about the intended press conditions that will be used at the printing stage and use this information to create a customized CMYK setup.
Photoshop CMYK myths
There are some people who will tell you that in their ‘expert opinion’, Photoshop does a poor job of separating to CMYK. I bet if you ask them how they know this to be the case, they will be stumped to provide you with a coherent answer. Don't let anyone try to convince you otherwise, because Professional quality CMYK separations can be achieved in Photoshop. You can avoid gamut clipping and you can customize a separation to meet the demands of any type of press output. The fact is that Photoshop will make lousy CMYK separations if the Photoshop operator who is carrying out the conversion has a limited knowledge of how to configure the Photoshop CMYK settings. For example, a wider gamut RGB space such as Adobe RGB or ProPhoto RGB is better able to encompass the gamut of CMYK and yield CMYK separations that do not suffer from gamut clipping. This is one big strike in favor of the Photoshop color management system. Remember, CMYK is not a one-size-fits-all color space and CMYK conversions do need to be tailor-made for each and every job.
CMYK setup
If you examine the US prepress default setting, the CMYK space says ‘U.S. Web Coated (SWOP) v2’. This setting is by no means a precise setting for every US prepress SWOP coated print job, because there can be many flavors of SWOP, but it does at least bring you a little closer to the type of specification a printer in the US might