Lightroom 4 includes long-awaited soft proofing functionality, providing an onscreen preview (the soft proof) of how your image will appear in print (the hard proof). The concept, if not the actual practice of soft proofing is rather straightforward. Monitors, by and large, are capable of displaying a wider dynamic range and color gamut than we can print. Simply put, there are image colors you can see onscreen that cannot be printed. If you can preview this mismatch before you print, you can make specific adjustments to the file destined for the printer to address these differences.
Previewing print output
In the Develop module go to View>Soft Proofing>Show Proof or use the keyboard shortcut (S) and your image is displayed in the main editing window surrounded by a 'white' background. In the soft proofing panel located just beneath the histogram, you can select any ICC profile that is installed in your system, choose between a Perceptual or Relative (colorimetric) rendering intent and immediately observe how these two parameters affect the contrast, saturation and brightness of your 'print' image.
In short, you are previewing the print output on your monitor. Checking the Simulate Paper & Ink box takes things to a more precise level of comparison. Instead of showing the white background as defined by your monitor (which will always be brighter and more neutral than printing paper), this option attempts to mimic the hue of the paper you've chosen via the ICC printer profile, applying it to the background and throughout the image area. The deep rich black of your monitor is similarly mapped throughout the image area to the relatively weaker tone of the ink/printer/paper combination specified in your selected ICC printer profile.
If all you could do was see just how much flatter and duller your print was going to look in comparison to what you see onscreen, soft proofing would be very depressing. Fortunately this is only the start. When you hit the Create Proof Copy button, Lightroom makes a virtual copy (VC), places it alongside your original image in the library and adds the profile name to the image's metadata (for easy searching).
Now you're free to make adjustments to brightness, contrast and/or saturation until this proof copy more closely resembles your original image. And because your changes are applied to a VC, your original image remains unaltered. Note that if you attempt to make edits in soft proofing mode without first creating a VC, Lightroom will prompt you to create one.
Although not soft proofing per se, in Lightroom 4's Print module you can adjust both brightness and contrast of the print output without altering your catalogued image. There is no visual preview of the adjustments so getting them right will require a fair bit of trial and error. If you constantly complain of prints that are too dark (or light), however, and are intimidated by the soft proofing workflow described above, this offers a quick way to adjust your print output without affecting your catalogued image.