Modern Printing and Post-Modern Printing
There are now many more authors than there are publishers of books. A strong force driving this disparity is that just about every school child owns and uses a home computer, but commercial printing presses have difficulty accepting copy from that source, especially photographs. The inherent mismatch may have something to do with commercial printing presses switching from Gutenberg's movable type to printing page images at least a generation or so ago, while computer printers took the direction of perfecting the Gutenberg method rather than replacing it. Let's give a simplified explanation.
Very few commercial printers do physical type-setting anymore. Since 1993 when Adobe invented the method, they generally work from what amounts to a photograph of each page, called a PDF or portable description format. To some extent, portions of a page can be stitched together like a patchwork quilt, but all of the pieces must agree on certain basic rules, one of which is that everything is printed at 300 dots per inch. That's essentially 300 pixels per inch.
By contrast, low-volume desktop printers can afford to take the time to examine each character or image as it comes along and readjust as appropriate. Essentially, computer printers do individual typesetting every time a new page is printed. They are thus able to exploit considerable compression for storage, or for the speed of electronic transmission. For them,72 dots per inch are sufficient, since computer-driven printers have acquired the facility to guess the gaps between dots well enough to fool the eye of the reader. Since the same thing is true of display monitors as computer printers, there is considerable resistance to bridging the remaining gap to the needs of the mass-printing industry.
Unfortunately, it is hard to convert photo images from 72 to 300 dpi. The best illustration so far offered is the washing of wool socks. You can throw argyle socks in a washing machine and they will shrink to the size of baby booties, but they won't stretch back up if you decide to wear them. The usual method attempted is to enlarge a small picture and take a second picture of it, then enlarge the enlargement, and so on. The Genuine Fractals program by Altamira Group comes closest to achieving the desired result, but even it has limits. When someone has more than a very few pictures that need stretching between Internet screen display and commercial printers, the best available advice is to store two different-density copies of the same image and use each as required.
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Slight re-design of work flow is advised, to include a third stored version of the image. The biggest possible image with the most pixels possible is stored on the author or publisher's own computer. A second, shriveled 72-dpi, image is sent to be stored on the host computer of a website, and used for desktop printing as well. When commercial printing is desired, the large stored image can be shriveled to 300 dpi and this third version of the image is incorporated into Office Word for editing, subsequently incorporated into a pdf file for final printing. Fine art display or other highly demanding graphics will follow this general outline, as well. Those who have any aspiration for high-density output would be well advised to go back to the original photograph. In 2008, that translates into the maxim: Always take all your photos in RAW format for permanent storage. Less demanding output can be produced from degraded copies of the original, the most common of which is now the so-called JPEG format. Those who discard the original RAW image, are almost always sorry.
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dpi (dots per inch) is not an inherent characteristic of an image ... it is an instruction given to a printer, screen or other output device.
An image consists of a rectangular matrix of pixels. The area described by this matrix defines the number of mega pixels in the image.
A typical raw image taken by a Canon 20D digital camera is:
Therefore, this image contains 3054 x 2336 = 7,134,144 pixels in total ... somewhat less than 8 mega pixels.
A color pixel with 8-bit color depth consists of three 8-bit bytes ... one each for Red, Green and Blue (RGB).
Therefore, the image in this example would take up 3 x 7,134,144 = 21,402,432 bytes on the disk ... 21 mega bytes
CRT monitors used to display the pixels in an image as dots projected from three "guns" in the back of the tube, one gun per RGB color.
Because of the limited resolution of the old CRTs, 72 dots per inch (dpi) became the default for images displayed on a computer monitor.
The image in the example, displayed at 72 dpi, would be
Because these dimensions are somewhat outlandish, and because 21 mega bytes is a lot to send down a telephone wire with a modem on it, images intended for display on computer monitors have traditionally been reduced in size by software like Photoshop. A 21 mega byte image might be reduced to 250 kilo bytes, a 98.8% reduction. The quality of an image that has been so drastically reduced is substantially degraded but the resolution of the monitor is usually so poor that you can't tell.
A typical JPEG image on Philadelphia Reflections might be 500 x 300 pixels in dimension, 7 inches wide by 4 inches tall at 72 dpi, which may be reduced a bit further by the width and height attributes of the img tag. Very often, these pictures started out life as much bigger images. They were shrunk to accommodate the inadequacies of the display devices and the transmission lines available.
With reflected light, R+G+B combines to create white; ink on paper, however, absorbs light and R+G+B ink creates a muddy-colored mess that only approximates black. For this reason printers do not use RGB, they use CMYK ... Cyan, Magenta, Yellow and blacK, which in a color-space sense are the inverse of RGB. Artsey fartsy printers these days actually use several different kinds of black to simulate the old silver images of Ansel Adams.
A museum-quality ink-jet printer creates images on paper by spraying dots from numerous jets in the print head. These printers are capable of laying down as many as 7200 dots per inch and many photographers who are concerned about the quality of their work print out at that dpi resolution. A resolution this detailed obviously requires a gigantic image file.
However, the magazine and book trades have settled on 300 dpi as the default.
The image in the example, printed at 300 dpi, would be:
Even this might be too large for the particular application, but the amount of further reduction necessary would be quite a bit less than was required for the monitor and therefore the quality would not be as degraded.
The 500 x 300 JPEG printed at this resolution would be:
As outlandish in its way as the raw image displayed at 72 dpi.
Another consideration is the distance from which the image will be viewed. A book is viewed from arm's length or closer; a picture hanging on the wall will be viewed from a further distance and a billboard from even further ... the bigger the picture, the further away you will stand to look at it.
The further away you are from the image, the fewer dots per inch you will need to achieve the same perceived level of quality; this is the trick of Pointillists like Seurat whose work up close is not recognizable as being what it is from a distance. So if you start with the raw image example that is 10 x 8 inches at 300 dpi and you want to blow it up to hang on your wall, you will have to up-sample the original image (using Genuine Fractals, more than likely) but you will be able to get away with 270 or 150 dpi, depending on how far away your audience will stand from it. The less up-sampling required, the less you will degrade the quality, because no matter how good Genuine Fractals may be you really are trying to create something from nothing and that will cause some level of distortion of the original image.