Color printers are similar to B&W printers, in that they must print several of the printer's dots for each image pixel (except for dye sublimation printers, which can make any color on any printed dot). Inkjets have only 3 or 4 colors of ink, a few have 6 colors, and this is all they can print. They CANNOT print any one of 16 million colors on any one dot. So to represent each image pixel in various colors, shades, and intensities, the image is dithered, meaning the printer uses a pattern of several of its dots to simulate the color of each pixel in the image.
For example, to print one "pink" pixel on our inkjets, we know it must mix some red and some white. There is no white ink, white is the bare paper color, no ink at all. To make red, the printer only has the CMYK (Cyan, Magenta, Yellow, and blacK) ink colors, and so must use a few magenta and yellow ink dots, not necessarily equal numbers of each, to achieve a certain shade of red. To make lighter shades of red, blank white space is used in the right amount. Black ink dots are used to darken some colors. The average visual effect of all these individual magenta and yellow ink dots, white paper, and sometimes perhaps black ink too, looks pink to us. But all of these multiple ink dots represent or simulate the color of only ONE pink image pixel.
So it is clear that we don't get anywhere near 600 or 720 or 1200 or 1440 dpi of "image" resolution from our printers in Color mode. This requirement for multiple printer dots for one image pixel greatly reduces the printer's real image resolution capability to a fraction of the printer's advertised dpi. Printer specifications are real and accurate and meaningful, but are not to be confused with image resolution. Printer ink dots and image pixels are simply very different things, and one color image pixel requires many printer ink dots. This is why we need a 1200 dpi printer (ink dots) to print an image at 250 dpi (pixels). And like B&W printers, attempting higher resolutions on color printers simply limits the pixel size area, allowing fewer ink dots, which then limits to even fewer possible color tones. We need the several ink dots in that space to simulate the correct color of the pink image pixel.
Inkjet printers do not use the four CMYK halftone screens from an image setter like commercial printing presses require. We never create CMYK images unless we are doing prepress for an offset ink press. There are unnecessary losses in RGB/CMYK conversion, particularly in the bright colors. Our inkjet printers are designed to expect normal RGB images. Their printer driver expects to convert RGB to CMYK ink, and uses dithering (stochastic with error diffusion) to produce the required color combinations from the three ink colors.
Dithering is the use of scattered dots, somewhat randomized instead of ordered halftone grids, which looks smoother on low resolution devices. The printer's limited combinations of three ink colors can rarely make the exact color for an image pixel. There is usually an error, a difference in the desired color of the image pixel and what the printer's dots of three colors can do.
Error diffusion means that the color error difference is carried over to four adjacent image pixels, one to the right and three below the pixel in error. Those next pixels are intentionally overcompensated in the opposite amount. If the possible dot combination for one pixel is not red enough, the next neighboring pixels are made overly red, so to speak. Then their own error term is carried over to their neighbors in turn. As this process moves across the image, compensating the color error, it all balances out and we see the right color.
Stop and think a second about what you see. Photo quality on an inkjet printer needs images around 250 dpi. However, video screen images are displayed on the monitor at about 75 to 100 ppi apparent size. Yet the video image usually looks better. The big difference is that every RGB phosphor dot on the screen can reproduce ANY of the 256 intensity values. But a printer's ink dot can only be either present or absent (two values). Inkjets must simulate pixel colors by using combinations of several ink dots of only the four CMYK colors. Inkjet printers are relatively crude devices, and instead of more spatial resolution, what they really need is more color depth or color resolution - they need a better way to reproduce the color of an image pixel in a very small space on paper. They can't use more pixels, smaller pixels simply limit even more their ability to accurately simulate the correct color of each existing image pixel.
Scale photo images to print at 240 to 300 ppi
This is a good guideline for inkjet printers (1440x720 dpi, 2880x720 dpi, 1200x1200 dpi, even 2400 and 4800 dpi) for printing Color or Grayscale photo images in high quality mode on good photo paper. There is not much benefit from the larger numbers, not when the ink dots are much larger than the grid to hold them.
Line art mode is the exception, being 1-bit 2-color B&W with no gray, no halftones, no dithering. In line art mode, the printer CAN use its full resolution, because pixels and ink dots can be the same spacing then. Every line art image pixel is either Black or White, and the printer can make black dots without dithering colors. You may want to print 600 dpi line art images, since line art is the exception, the one mode when the printer can use an image resolution equal to the ink dot resolution of the printer.
But for Color or Grayscale modes, image pixels and printer ink dots are very different concepts. Photo images at 200 to 300 ppi are very appropriate for inkjet printers. Generally about 250 ppi is ideal on photo paper for color or grayscale. 300 ppi may be slightly better at times, but it will be difficult to see, and even 150 ppi images might sometimes be acceptable, for larger images, or for plain paper.
Graphic images (including images of text) need higher resolution than photo images, probably 300 dpi for commercial work, to maintain sharpness of the hard edges. Line art is best at 600 dpi.
It used to be that the lower inkjet printer dpi rating number divided by 3 or 4 was a very crude approximate range for printed images, but it loses all meaning for today's inflated 2400 and 4800 dpi numbers (the ink dots are simply much larger than these grid locations to hold them).
See the chart on page 86 for guidelines for other printing cases. Scaling to more printed resolution than that won't be very helpful. Often we can only print what is convenient for the image size we have, and if that resulting number comes out at say 187 ppi, then that's just fine, there's no alternative for that image and that printed size. Paper quality varies too (ability to retain small ink dots), and 150 ppi is plenty for plain paper.
Images vary too, some are sharp and some are not. Sometimes more than 150 ppi won't help much. The low-end of the range is often fine, experiment to see if you can tell much difference. Giving the printer huge images can be pretty slow, it takes the printer time to receive and discard all of those pixels. Note however that if you are printing the printed image larger than the original photo, then the scanning resolution should be increased accordingly, in the same ratio as the image size (see Scaling in the previous section).
Inkjet printers have come a long way in the last few years, and the once impossible photo quality (not very long ago) is taken for granted today. The 6-color photo inkjet printers became the state of the art for printing photographs at home. They add light magenta and light cyan ink to provide more choices to help color accuracy, and specifically to help hide the ink dots in the light areas. These photo printers in some cases can take advantage of higher image resolution. The current best inkjet printers also have smaller ink dot drop sizes available, and these also help simulate the lighter colors. 6 ink colors is very good, but less important on the newest printers today, they are all great.
Epson always suggested printing images at 240 ppi for maximum detail on the good photo paper. Some people say they see improvement at 360 ppi on the best new 6-color models, but I can't say I can detect this on an Epson 780. Perhaps we are looking for different things (see page 98). Experiment in the 240 to 360 ppi range on your sharpest images. Many of our snapshots really don't have maximum sharpness, and don't need maximum resolution, but some images certainly can use more, and up to 300 ppi is a good routine goal for photo paper. But 300 ppi is not a "gotta have it" goal in every case. 240 ppi is likely about as good.
HP REt ink dots are 600 dpi, but in REt mode, HP uses a unique technology that blends colors by printing several ink drops on any one dot. A pixel's color value is dithered on the same one dot location instead of by a group of dots. Some of these possible combinations are murky, but each dot can still make many useful colors (instead of one color) and error diffusion dithering still corrects any error. It is not continuous tone, but it's rather close. To an extent, printer dots and image pixels can be matched one for one, like on a monitor but with higher color error. Printing inkjet images at 300 ppi is sometimes reasonable, but you have to look mighty close to detect any difference over 250 ppi.
You'll hear many strange things on the internet, including about how some users claim to scan and print color photos at 480 ppi, 600 ppi, even 1440 ppi, and WOW, do their printed images ever look better. Yeah, sure they do, but don't believe everything you hear. It is very advisable to test this yourself, because inkjet printers don't have nearly that much photo image color resolution. The good commercial dye-sub printers (tens of thousands of dollars), and the Kodak and Fuji optical printers (laser light onto silver halide photo paper, chemically developed) want 300 ppi images. 300 ppi is plenty for humans. What we see on paper is generally less about the image resolution, and more about the printer's ability to actually reproduce those pixels. It's a real stretch to imagine that an inkjet printer can use 600 ppi color images.
Do experiment to satisfy yourself that 240 to 300 ppi is reasonable to give you all the photo quality there is to get, so you don't needlessly waste time and memory or disk space scanning with higher resolution that is not useful. Giving the printer excessive pixels can even be detrimental to the printed image, depending on how skillfully the printer discards the excess.