Practical Color Management: Eddie Tapp on Digital Photography

3.3. Output Stage

Output is perhaps the easiest stage with regard to maintaining consistency. The challenge is to calibrate and profile your devices and maintain them. The ultimate goal of any digital workflow is the final product. You want to make sure that the image is printed or electronically displayed correctly.

Initially, calibrating some output devices may require a fair amount of work, but once calibration and profiling are established, they're easy to maintain. You may be outputting to an ink-jet printer, a dye sub, a photographic lab, a toner-based color copier, or a printing press. Or you may be outputting to the Web.

3.3.1. Preparing a File for Output

Regardless of the final destination, there are three areas of transition that your file may need to go through during the output phase: resolution, sharpening, and color space conversion. These tasks can be performed manually in Photoshop or automatically within a driver or software known as a RIP (Raster Image Processor). (Rick will discuss RIPs in detail in Chapter 5.)

Resolution

Take, for example, a 36MB file that you've processed and are now ready to send to your photographic lab. The resolution from this file is easily suitable for a 16 x 20 size print with little or no resolution changes. If the file is going to be printed at 30 x 40, then the file resolution must be resampled to match the set resolution of the printing device, which could require the file size to rise to 80140MB. Resampling can be achieved in Photoshop under Image Image Size.

Sharpening

Sharpening an image requires an increasing contrast of pixel data, thus removing pixel data in the process. Because of this data loss, sharpening for output should be one of the very last steps and established for a particular output size. (See the section "Sharpening" later in this chapter.)

Converting color space

Converting the color space of the file to the color space of the printer is when color management is truly performed.

The good news is that, in most cases, when you send your 36MB RGB file to your lab, it will use a software RIP that automatically performs these three important steps.

With an in-house workflow to your own output device, you have several options to automate the output stage. You can create an action in Photoshop that will resample the image (if needed), sharpen it, and bring up the "Print with Preview" window, at which point you can select the output profile for your ink/paper combination and then establish the print driver interface. Or you can use a software RIP to apply these same options (more on this in Chapter 4).

3.3.2. RGB or CMYK?

Output devices are either RGB or CMYK. Ink-jet printers use CMYK inks (and sometimes light CMYK inks). However, when you send a file to an ink-jet printer, send RGB files and the print driver or RIP will disseminate how ink will be applied to the paper.

Unless you are preparing files for a printing press that requires CMYK files, such as files for a magazine or newspaper, you are working in an RGB environment and your files should be processed, saved, and archived in RGB mode. If your RGB file needs to go to a printing press, then converting to CMYK should be included as one of the last steps. If you do not have an ICC profile to convert to or, even then, if you are not familiar with the properties for converting to CMYK, ask for prepress assistance from the printing plant or a press ICC profile to convert to, or let the printing plant do the final CMYK conversion to avoid potential conversion or press problems. (More details for this step are covered in Chapter 4.)

3.3.3. File and Printer Resolution

Resolution is a subject that confuses many people. There are all sorts of rules out there about what is best. The best way to understand resolution is to get a general understanding of how it works and then test it yourself.

As we learned in the "Input Stage" section of this chapter, input resolution is defined in pixels per inch (ppi). An image is no more than a file that has both a width and a height consisting of a certain number of pixels. A file does not consist of dots per inch (dpi). The dpi is relevant only when you go to print your image, and that distinction is the source of much of the confusion about resolution. If you size a file for output, first set size in inches and then set the resolution that conforms to the output resolution. This information is placed in the header of the file so that applications and output devices know the size that you want to print to.

For example, most ink-jet printers will print anywhere from 150 to 2880 dpi. Do you need to print at the highest resolution? It depends on many factors, including the printer, the paper, screening, the speed of printing, and the often overlooked issue of viewing distance. Let's look at each of these issues:

Printer resolution variation

Different printers have different resolutions, and even though one printer has a higher resolution than another, the quality of the output is the main thing to consider. As with camera megapixels, printer dpi is often a marketing number (read: hype). A higher dpi printer will print more dots per inch, making the print appear smoother. Most ink-jet printers now print with additional light inks for some of the colors, and this helps make the print look smoother or more continuous.

Paper choice

Papers range from high gloss photographic to fine art rag, with a variety of speciality papers in between, including canvas and backlit. Ink-jet printers do no more than spray ink out of a nozzle onto paper. When the ink hits the paper, each type of paper responds to the ink differently. High-quality glossy papers accept the ink without spreading very much. Matte rag paper, on the other hand, spreads the ink out. For this reason, you can often print at a lower resolution on a matte surface paper than a high-gloss paper because the spreading ink suppresses the dots.

Screening

Screening is the way the dots are laid down on the page. Ink-jet printers use several types of screening, including stochastic and error diffusion. The details aren't importantjust remember that screening can affect how the image looks on the paper.

Print speed

Speed is very much affected by the resolution at which you print. The higher the resolution, the slower the printer. If time is not an issue, you can print at the highest resolution. On the other hand, if you are trying to produce a lot of work in a limited time, you may want to consider printing at a lower resolution.

Many people think that if you print at a lower resolution, the color is not as good. For the most part, with proper color management, the color at higher resolutions should match that of lower resolutions.

Viewing distance

The rule of thumb is that the greater the viewing distance, the lower the dpi needs to be. A 4 x 6-inch print generally needs to printed at a higher resolution than a 30 x 40-inch print because the latter will be viewed a greater distance.

Ultimately, the goal is to send the smallest file size to the device that yields the best results. Keeping the file sizes smaller will speed up your workflow and save hard drive space. Here is a starting point for setting output resolutions for various devices:

• Ink-jet: 150 to 360 dpi

• Photographic paper: 200 dpi

• Dye sub: 150 dpi

• Printing press 150 lpi (lines per inch): 250300 dpi

The best way to determine the optimum resolution for your printer is to test it yourself. When you have a healthy file (meaning good tone and color values), you gain what I call the "forgiveness factor" when it comes to getting the most out of a printer from the file resolution. Once again, good exposure and white balance can go a long way. I've printed images as large as 40 x 60 inches on an ink-jet printer from a 50MB file with superb results.

3.3.4. Sharpening

In a basic sense, sharpening is increasing contrast (removing pixel data), especially around contrasty edges, thus emulating a sharper image. The more pixel data that is resident in the file, the more sharpening you can apply with excellent results.

With the exception of processing a RAW file within a RAW-processing application, processing a file to achieve better color and tonality takes the pixel data and removes it, thus creating weaker pixel data but improving the visual appearance. This is also the case when you're sharpening an image.

Three factors come into play for sharpening a file for output: the type of the original image, the output size of the image, and the device that is printing it.

Type of image

An image such as a landscape generally requires more overall sharpening than a portrait does. Images sometimes require selective sharpening in different areas, such as sharpening of the eyes and lips of a portrait.

Output size of the image

To maintain the quality of an image, sharpening must be preformed after resampling in Photoshop. Sharpening an image for an 8 x 10 would require settings different from those required for a 30 x 40-sized image.

Output device type

Some output devices require more sharpening of the image than others. Such is the case with a printing press because when the ink is placed on the paper, the ink spreads out. This is called dot gain (take an ink pen and touch it to paper, and you'll see the ink spread). Dot gain causes the image to print slightly softer than the original image. Because of dot gain, you might need to seemingly oversharpen the image to get the right sharpened look when the image is printed. An ink-jet printer is considerably different from a printing press, so you will need to experiment to achieve the proper sharpening.

Some output devices have software that can sharpen an image for the size being printed. With this in mind, when you send files to an outside vendor, such as a photographic lab, check with the lab regarding the level of sharpening needed, if any. Again, as with an ink-jet printer, you will need to experiment with sharpening at different degrees to find what works best.

There are many techniques for sharpening images in Photoshop for output, including the Unsharp Mask and Smart Sharpening filters. The Unsharp Mask filter offers controls for the percentage of sharpening, a pixel radius to control edge contrast, and a threshold to smooth non-edge areas. The Smart Sharpening filter has additional controls to minimize Gaussian, Lens, or Motion blur, and an Advanced button that allows you to control the blending of edges where you might have a halo effect (which is normal when sharpening).

3.3.5. Preparing Files for the Internet

Preparing files for the Internet is one of the more difficult challenges during the output stage because you really don't know how the audience's monitors may (or may not) be calibrated and profiled. On the other hand, outputting images for the Internet is certainly easy to regulate within a color space. And it is possible to use a system profile (monitor profile) to funnel a space of color before converting to the final output color space with certain types of image files, such as monitor screenshots.

Considerations for preparing files for the Internet include downsampling the file to specific pixel dimensions rather than a file size, sharpening (which we discussed earlier in this chapter), and setting color space:

Downsampling

In a general workflow, large files can be resampled in Photoshop using Image Image Size. With Resample Image checked and Bicubic Sharper selected, you can then type in the actual pixel dimensions and select OK, and the image will downsample and sharpen for the smaller size.

Setting color space

A file color-managed for the Internetshould be kept in a small color space, such as sRGB. Convert the color space in Photoshop under Edit Convert to Profile; select sRGB if the file is in a different color space. Note that tagging a file with a color space such as sRGB will create a larger size file than one with no color profile. If this is an issue with images you are uploading to the Internet, you can choose not to embed a profile to keep the files as small as possible.

Occasionally, when an image color appears way off on the Internet, funneling the color space first into your calibrated monitor's profile via Edit Assign Profile (and selecting your monitor profile) and then converting via Edit Convert to Profile and selecting sRGB may funnel the color into a proper viewing condition.