By Lou Dolinar
Last in a series
Updated Feb. 14, 2006

While every printer is different, it helps to know what's going on behind the scenes.

First off, forget everything you thought you knew about your printer. While the "dpi," or dots per inch, that manufacturers typically quote bears some relationship to print quality (my HP Photosmart 8450 is rated at 4800 by 1200 dpi), it is not the same as the pixels per inch a monitor displays, or the millions of pixels advertised as "x mega pixels" for your digital camera, or for that matter, for the dots that make up the pages of National Geographic magazine.

What's the difference? On screen, in a digital camera or scanner, or in National Geo, one dot can represent any one of millions of colors. When we look at one of these images composed of pixels, our eyes perceive the accuracy of the color, not the number of dots. If the color is all there on a printed page, as few as 200 dots per inch looks "natural," although generally 240 to 360 dots look a touch better. Photos on your color monitor look fine at 72 dpi.

Ink jet printers can't create millions of colors directly. They have a paltry four to nine colors in cartridges. To create other colors, ink jets lay down microdots to create the individual 200 to 360 per inch macro-dots provided by the camera or scanner. The way the micro-dots are blended is called dithering. The macro dots themselves are dithered as well. Some firms (HP being one of them) also may shrink or enlarge the microdots to further refine the image. All the above is done in software, and thus the quality of the driver is at least as important as minor differences in resolution.

What's this mean to you?

Well, don't, as most novices do, sweat it because the pixel density of the your scanner or camera doesn't live up to your printer's, or use giant files in the hopes of better reproduction. The basic rules we gave you the first week of this report still hold: Set your scanner for 240 to 300 pixels per inch. For digital cameras, you'll find some excellent charts of how many pixels you need for different sized prints at Popular Photography's Web site:

What if you don't have enough pixels to print an enlargement? You'll get a picture, but it will have "jaggies," or blocky distortions. Photo editing software, through a process known as interpolation, will allow you to roughly double the size of the print by minimizing these artifacts. It's nowhere near as good as the real thing, but from a distance, not bad. You can further improve the quality by using the "unsharp mask" function in your image editing software.

Calibration is another issue that you may need to address. If the color cast of your prints differs greatly from your screen display, you probably need to calibrate the two. (Typical cause of the problem: You boosted the brightness and contrast of the monitor to make up for a bright room). The way you do this differs somewhat among brands, but the basic idea is to adjust brightness, contrast and color of the monitor to a standard, either that of the ICC (International Color Consortium); or that of your software (Adobe's is widely supported), then adjust the printer to match the program.

In general, you'll want to use ink and paper from your printer's manufacturer. Nothing magical here - paper, like monitors, can alter color fidelity. When you use your manufacturer's paper, you're using paper that's calibrated to the printer and the specific paper option in the print driver, which, in turn, you're calibrating to match the display.

Want to use a nonstandard paper? Some paper manufacturers are offering calibration software for their paper - you load a driver into your PC, and it shows up on the menu with the manufacturer's paper. Alas, paper calibration drivers are still a relative rarity, which is why photo paper is so expensive. I have yet to hear of any drivers for after-market ink products, which is probably why their use for photo printing is considered dubious.

Regular cleaning and alignment helps quality. Most printer software gives you the option of cleaning the print head, and of aligning the cartridge automatically. Suspect the latter problem when you see lines or other artifacts in your prints. Also check your manufacturer's Web site for manual cleaning recommendations - my HP, for example, likes to have its cartridges gently hand- swabbed.

Drying time and longevity of prints vary widely among brands, so check your manufacturer's Web site for specifics. In any event, you ideally should remove each print as it is completed to keep the next one from smudging it. I've seen recommendations for as long as seven days drying for prints that are being placed under glass.