After two parts discussing colorspaces and printing processes, we're going to get into actually preparing a digital photograph for printing. Part 1 covered the two worlds of color we photographers deal with: RGB and CYMK. Part 2 covered various printing processes. Keep the material covered in parts 1 & 2 in mind while prepping your images.
Equipment & Environment
There are a few things to take into serious consideration when doing your own color-correcting (post-processing) or printing. These considerations apply whether you use a printing service, have your own printer, or just publish to the web. They'll help you produce accurate and consistent results.
The piece of gear you simply must have to accurately process your images is a calibrated monitor. Of course, it doesn't have to be a $2000 Eizo or something like that, but your monitor, be it a laptop screen, CRT, or LCD needs to be calibrated for color, contrast, and density. Without a calibrated monitor, your color, density, and tonal corrections are being done blindly. You can't make corrections by eye and have faith in their reproduction.
If you're using a printing service or professional lab as I do, then you'll really only need a monitor calibrator. I use X-rite's Eye One Display2 ($250) which enables me to calibrate my RGB devices. If you're doing your own printing, I suggest using X-rite's ColorMunki ($400) to calibrate your monitor and printer. There are other calibration systems (spectrophotometer) out there from various companies such as Pantone, Datacolor, and GretagMacbeth with a wide range of prices and options depending upon your needs.
The next thing is your post-processing environment. For best results, your room should conform to the ISO standards for digital photography and your particular printing process. However, this isn't always possible as it means your room needs to be of a certain color and luminance. For the rest of us that aren't doing what many printhouses need to do, a room that has a consistent lighting situation and neutral colors works great. How bright or dark a room is and the colors of the walls all influence your ability to see color accurately.
For example, my color-editing area is in the basement where I have control over the lighting so I can better hit the sweet spot of those pesky ISO standards. However, my walls are a pale yellow and I'm not about to move furniture, etc. to repaint it. So, to counter that I installed some daylight-balanced lights. This counter-balances the yellow walls somewhat, neutralizing the colorcast. I also have my light at a distance and angle that doesn't influence my monitor much.
To further cut down on my ambient's influence upon my monitor, I use a hood. This helps shield glare which not only affects color, but mostly density. The hood isn't really necessary, but every little bit helps when you're trying to faithfully reproduce what you see on screen.
Garbage In, Garbage Out
Before you pop your image into Photoshop or your favorite image-editor, remember the phrase, "garbage in, garbage out." If your photo is crap from the beginning, no amount of post-processing, expensive printing, or prayers will save it. This also means that if you provide low-quality data, paper, ink, and/or printing at any point in the process, you'll degrade your image. Keep in mind that as you process your image, you're moving away from the original — you're throwing away pixels. So, give yourself the best foundation by taking a good photograph and operating at as high a quality as possible throughout the process.
So, we're now ready to make our amazing photograph ready to be amazingly printed. Assuming you've calibrated your devices properly and have the ICC profiles for your printer/ink/paper combination all set, we're ready to drag that image into Photoshop and tweak it to fit nicely within the gamut of your output device.
Firstly, if you're shooting in the RAW format, drop that file into your RAW processing program, trying to keep the colors and density from going outside the histogram. Clipping highlights or shadow detail affects your ability to color-correct as well as print with detail in the extremes. When processing your RAW file, keep in mind not to pump up your contrast, density, or saturation too high because we'll be soft-proofing within Photoshop.
Once you're done pre-processing your photo, export it in Adobe RGB and 16-bit color, TIFF.
Adobe RGB has a larger gamut than sRGB so you have more room to play. 16-bit color give you more depth than 8-bit. The TIFF file format is more robust than JPEG, especially for heavy image manipulation. This translates to more leeway, better color, and smoother tonality.
The downside to TIFFs is that the file size is much larger than a JPEG due to the low compression of the data. So, if your computer can't handle it or you want to save some disk space, the JPEG suffices. Besides, you took a great photo to begin with, right?
I used to do a lot of 8-bit, AdobeRGB JPEGs and thought the colors were great. And they are, but when compared to 16-bit things get smoother and better. 8-bit color is perfectly suitable for digital viewing, but for [high quality] printing, not so much. At first a 16-bit image and 8-bit image look the same, but once you start throwing away pixels (editing) the 16-bit image can withstand it dramatically better.
The 8-bit image falls apart much faster than the 16-bit image.
We've opened up our high-quality image in Photoshop and now we're ready to optimize it for our specific output (web, print, etc.). Let's set up our workspace to get a better sense of the changes being made. Not only do we need to visually judge the image, but also having the numbers to work with is helpful.
In Photoshop, or your favorite image-editing software, open up your "Info" panel (F8 in Photoshop). The Info panel displays a bunch of important information, hence the name. It shows your cursor's position, dimensions of objects and selections, as well as color values. For this tutorial, the color value information is the most important.
Another important piece is setting up your soft-proofing. In Photoshop, go to "View > Proof Setup > Custom..." A window will pop up with a few drop-down menus. Select your output device's ICC profile, and in the other menus, select the options needed to properly render the soft-proofing. By "output device" I mean the paper-ink-printer combination you'll be using. If you use a professional printing service, they'll provide you with the color profiles as well as the rendering intent and display options. If you've made your own, following the insructions for that setup.
Saving frequently used soft-proofing settings makes things move more smoothly.
Finally, when you open your image, make sure you're working in 16-bit mode if your working with a 16-bit image. Be aware that some options in the "Filters" menu aren't available in 16-bit mode. You can temporarily switch to 8-bit mode but be sure to go back to 16-bit whenever you're proofing, saving, or exporting for print.
So, now that we have our workspace established and our (pre-processed) photo, we're ready to begin making those adjustments to prepare it for printing.
Optimizing the Image
This part is somewhat simple but it is important because here we'll actually make the changes necessary to get as much usable data (color, detail, and density) out to the printer. If you're outputting to a high-quality printing method, you'll have a lot more room to make adjustments. For low-quality printing, you're going to be very constrained and take the best you can get.
The original image. No adjustments.
Since no printing method can match the RGB color gamut, we need to know the limits of the printer/printing method you're targeting. As a rule of thumb, giving your printer CYMK values between 90% and 10% is enough wiggle room for most printers to preproduce color well. Remember, that dot gain plays a factor in rendering color, detail, and density. And since dot gain is a percentage, you need to factor it into your calculations.
Important information to know is the maximum ink per color plate as well as the total ink for the printer, in percent. Some printers can do more than 100% on any given color plate. However, the total ink output may be cut-off at 300%, causing things to get clipped. So, the objective is to get as close to those limits without going over while being mindful of dot gain.
All this information (RGB and CMYK values) can be found in the Info panel and will give us numerical guidance alongside our visual observations. We'll use our eyes to walk in the adjustments, then use the numbers to fine-tune it.
What I like to do is open up the Curves adjustment layer and set my highlight and shadow limits there. This is done by setting my white point and my black point. I go and find the brightest neutral area with some detail in it and find the darkest neutral with detail too. Using the eye-dropper tool, I shift-click those two designated areas so that I can accurately return to them for reference. You'll see each point's color information and X,Y location recorded in the Info panel.
In the Curves tool, drag in the highlight end point to 247 and the shadow end point to 7. This range is a good general rule of thumb because most good printers can print detail within that range. Once that is done you'll notice that some clipping has occured and some color cast has been removed. The change may be minute, but you should see some changes in the RGB values you sampled earlier.
The image after delimiting to 7 and 247. Barely any change occurs in this photo because my actual shadows are around 20 and my highlights around 235. However, some color cast has been removed.
To preview how your print will look, hit Ctrl+Y (Cmd+Y for Mac). This will simulate your output device and provide you with a softproof. Your image should change in appearance, usually by colors and density flattening. To find out which colors are out of gamut, turn on your "Gamut Warning" with Ctrl+Shift+Y (Cmd+Shift+Y for Mac) and it will highlight which colors are outside the output device's gamut.
This information is really helpful in determining if you'll need to do a spot color plate or if you need to adjust that specific hue's density or color to bring it back in gamut.
Adjust your color, density, saturation, and other edits to try and fit within your softproof's gamut, toggling your previews until you're as close as you can get. Once all that is done, look at your white point and black point. If the RGB values aren't equal, they aren't neutral and any remaining color cast will only intensify when printed. Use a separate Curves or Color Balance tool to equalize those numbers as best you can.
I went into each color channel in the Curves tool and moved points around until I got R=G=B in both my highlight and shadow samples.
I added a Levels layer to increase my denisty and further neutralize color casts. Despite the dramatic increase in contrast, the 16-bit file held up great. Since I was printing to traditional photo paper, I could be more aggressive with my adjustments.
The last two things you'll want to do are work on approaching the maximum ink densities of your output, keeping in mind dot gain, and sharpening. For the densities, you can adjust saturation, levels, and/or curves to help mitigate the flatness of your image. Softproof often when doing this. Finally, apply sharpening after you've resized the image to it's final output size. Doing it this way ensures that the sharpening you see on screen is the sharpening you'll get in your print.
Once you're happy with your work, be sure you're in 16-bit color mode and your softproofing preview is toggled off. Save a copy of the file in a high-quality format such as a TIFF. You can use this TIFF as a master copy from which you can derive various sizes, only needing to apply the proper sharpening for each resize.
Finally, it's off to the printer!
Preparing your images for printing can be a lot of work, but if you get as much of your picture right in-camera some of these steps can be done extremely quickly or may even be skipped. Sometimes it's just a few minutes per picture, other times it's a lot longer. A good exposure goes a long way in giving you more control and saving you a lot of time. Good preparation (calibrating and profiling) saves you time when printing, too. Good luck with your next print job!