Printer Resolution versus Image Resolution
As we saw last week, images made by digital cameras and scanners are comprised of tiny pixels. Inkjet printers create images out of tiny drops of ink. While these may seem quite similar, there are some important differences.
Printers and monitors produce color in opposite methods. A monitor screen starts out being black and gets brighter the more color we add using a system known as additive color. If red, green and blue are at maximum, we get white. By contrast, printer paper starts out white and gets darker the more ink we lay down on it. This is called subtractive color.
In 8-bit mode, colors in an image can have red, green and blue values ranging from 0 through 255. Multiplying that out gives a total of 16 million possible colors for each and every pixel. Opening up the front cover on my Epson 4000 printer though reveals only eight ink cartridges: cyan, light cyan, magenta, light magenta, yellow, photo black, matte black, and light black. Each tiny printer dot is made from only one ink color. The printer does not mix inks so in order to represent all those different colors, it employs two basic techniques.
First, it varies the size of the ink droplets used to print. The smaller the droplet, the lighter the color will appear since more of the underlying paper will show around it. The Epson 4000 for instance can create dots as small as 3.5 picoliters (a picoliter is a trillionth of a liter, or 0.000000001 milliliters). Second, and most importantly, it builds up the appearance of varied colors by using arrays of individual droplets. Newspaper photos are printed in much the same way. If you take a magnifying glass and look closely, what at first seemed like a continuous tone image is in fact composed of a pattern of dots. If all of the dots in the array are printed in yellow, the combination will of course look yellow. If every other dot is yellow but the rest are magenta, you'll get red. Yellow plus cyan gives green. To make light green, the dots have to be printed smaller or some of them must be left unprinted so the paper shows. And so on. By varying the percentages of each ink color used as well as the size of the droplets, the printer can give the appearance of being able to create the wide variety of colors needed to make a photograph.
To minimize the appearance of dots, newer printers add light cyan, light magenta and other colors to the basic CMYK ink set to permit more even ink coverage while still being able to create light colors. Regardless of the ink colors used though, the basic dot strategy remains. Early inkjets often created visible dithering especially in areas with low ink coverage. Droplet sizes these days though are getting so small that they often blend together on the page to the point where they aren't noticeable at all. But they are indeed still there.
The obvious consequence of this method of simulating continuous color is that it takes a lot of tiny dots on the page, each made by a tiny droplet of ink from the printer, to represent each image pixel. 300 pixels per inch is generally regarded as the standard when preparing images for printing yet current desktop inkjet printers are capable of printing at 1440, 2880 or even more dots per inch. While these sound similar, they aren't. In quoting printer resolutions that high, Epson, HP and other printer manufacturers are counting each and every one of those tiny ink droplets. Obviously doing so makes their printers sound more impressive, but don't be fooled into upsizing your image resolution to those astronomical values. If you have enough memory and disk space on your computer you certainly can print at 1440 pixels per inch, but your prints won't look any better for it. They'll just take a lot longer to print since you will be sending a heck of a lot more info to your printer. Drops this small mainly let them create color better and only have a small impact on actual resolution.
Simulated printer output showing the dot pattern.
The image on the right really is a close-up of the image on the left.
Actual scanned printer output
from an early model Epson printer
So how much resolution do you need to get the most out of your printer? As mentioned, 300 ppi is often quoted as a standard resolution for printing, but it's not cast in stone. Many users report excellent results from resolutions as low as 240 ppi or as high as 360. Anything below this range may well yield slightly soft images while using anything above it isn't likely to provide any additional benefit. One trick that used to be common was to print at 288 ppi since this divides evenly into the base 1440 printer resolution, making the printer driver's job of interpolation somewhat easier. I honestly don't think it makes that much difference these days since drivers have continued to improve over the years and can create excellent results from most any input resolution between 240 and 360. Your results may vary of course, so try some test prints to see what you think.