WHEN RASTER IMAGES INCLUDE TEXT
Sometimes text characters are used in a raster image in ways that are primarily decorative or artistic. If text characters are not primarily intended to be read as information, they require no more resolution than any other raster image content.
But informational text is another matter. While resolution of 300 dpi is ideal for raster images, many presses can reproduce text at 2400 dpi or higher! Fonts behave much like vector images on press. The only limit to their resolution is the printing device. So type generated by a font is much sharper than it can possibly be in a raster image.
For informational text, you will attain far better results if you create the type separately in a page layout, word processing, or vector drawing application, then superimpose the text on the imported image.
Alas, real-world circumstances do not always allow for best practices. Sometimes designers are directed to use an elaborate image — combining text and art — which exists only in raster form.
If the image is available at effective resolution approaching 800 dpi, a typical reader will discern no difference between rasterized type and type generated from a font. Sometimes resolution as low as 550 dpi will yield "good enough" results. But be aware that type — if it is to be read as information — requires higher resolution.
AVOIDING COMMON MISTAKES INVOLVING RESOLUTION
Problems arise when people are not aware that printing presses require much higher image resolution. Without considering dpi, they choose an appealing photo that looks fine on their computer screen. Often enough, the lack of resolution is not apparent on inkjet or laser proofs. Some of these devices use no halftone process at all; others use a linescreen that is so low that it does not emulate what takes place on-press. Only when the job goes to the printer, does it become apparent that the resolution is far too low. The good news is that you can avoid such problems by making sure from the start that images are of adequate resolution.
BENDING THE RULES
In the real world, there are going to be situations where a preferred image does not meet standard guidelines with respect to resolution. What are your options?
Start by consulting your printer. In many cases you will find that the linescreen is the typical 133 lpi, as opposed to the premium 150 lpi. This brings the target for effective resolution down to 266 dpi.
Over the years, I've often enough seen people "cheat" by going with marginal resolution. If the results weren't great, neither were they catastrophic. When working with a digital press, a test run can often be arranged. But where offset printing is involved, be aware that effective resolution below 85% of the target standard is the province of roulette, where no one can guarantee acceptable results. As a general rule, images where a clear principal subject has well-defined, linear edges will fare better than those involving more in the way of rounded shapes and subtle details.
Photoshop can artificially increase the resolution via the Image Size function. Some among the digital cognoscenti warn against using it at all; others condemn it as sorcery. In earlier versions of Photoshop, this utility created new pixels by interpolating values based on bordering colors. Blurred edges and lost detail were often the result. The Bicubic Smoother option, introduced in 2003, was a significant improvement. Used sparingly, it can yield better results when "upsampling". But be prepared to invest some work in manually finessing the image.
An application called Perfect Resize 7 offers a better alternative. The software uses sophisticated fractal algorithms, and does a remarkably good job of upsizing images that have well-defined edges. Though no panacea, this application is the best option for those who require a large increase in resolution. A free trial version is available.
