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Let’s trigger a stick of gelignite under our digital images. And maybe explode some myths.
You’ve got a digital camera. Perhaps you’ve got an inkjet printer – one with great photo quality reproduction. Now you’re ambitions are beginning to take wings and you want to get into print with your images.
The razzle with digital cameras is that you can shoot pictures in seconds, drop them onto your computer’s hard drive, dibble with them via your favorite software, add a little text – then output them onto a sheet of paper.
So far, so good, if you want only a few copies. But, as we all know, inkjet printers – for all their magnificent qualities – are still costly, in price per print terms, aside from being slow. If you have in mind a run of a thousand copies you have to knock on the door of a printer to arrange comparatively large print runs.
Without getting into the technical complexities, the printer will take your digital image, make colour separations (usually four – CMYK – Cyan, Magenta, Yellow and Black), then go through the process of making printing plates and finally arrange the impression of four colour inks onto paper.
Unlike traditional film, the printing process is not a continuously variable process when reproducing tones – instead, a system of line screens is used. These screens break up the image into a series of dots in C, M, Y and K. The merging of these dots in the eye produces the final semblance of full colour on the printed page.
Printed output can be made on differing qualities of paper – according to your needs. Newsprint is at the lower end of the scale, with paper of the quality used in this magazine at the higher end.
Most magazines use a screen ruling of 150 lines per inch which produces a very high quality reproduction of digital and traditional film images – along with clear, crisp text.
The only problem is that digital camera images have defined limits when it comes to reproduction on paper, using a half tone line screen.
Let’s explain the problem another way by quoting from an Adobe manual.
Bear in mind, a digital camera image is a bit map image, made of computer created bits of varying density. We’ll deal in measurements in inches, for that is how the publishing industry still works.
(Reprinted with permission from Adobe)
About resolution and image size
Several concepts are important when discussing the characteristics of bit map images: pixel dimensions, image resolution, output resolution, and screen frequency. Another type of resolution, called bit resolution or pixel depth, is important when considering how color is displayed onscreen.
Every bit map image contains a fixed number of pixels, measured in pixel height and pixel width (the number of pixels displayed along the height and width of the image, respectively). The total number of pixels determines the file size, or the amount of data in the image. Pixel dimensions, along with the size and setting of the monitor, determine how large an image appears on-screen. A 13 inch monitor displays 640 pixels horizontally and 480 vertically. Larger monitors can usually be set to display a varying number of pixels, for example, from 640 by 480 pixels, at which setting the pixels maybe quite large, to 1920 by 1080 pixels, at which setting the pixels are small.
If you’re planning to display an image online (on a Web page, for example), your maximum image size is determined by the lowest pixel dimensions of the monitors used to display your image. For example, if your audience will view your image on a 13 inch monitor, you will probably want to limit the size of your image to 640 by 480 pixels.
The number of pixels displayed per unit of length in an image is called the image resolution, usually measured in pixels per inch (ppi). An image with a high resolution contains more, and therefore smaller, pixels than an image of the same dimensions with a low resolution. For example, a 1×1 inch image with a resolution of 72 ppi contains a total of 5184 pixels (72 pixels wide x 72 pixels high = 5184). The same image with a resolution of 300 ppi would contain a total of 90,000 much smaller pixels.
Because they use more pixels to represent each unit of area, higher-resolution images can usually reproduce more detail and subtle color transitions when printed than lower-resolution images. However, once an image has been scanned or created at a given resolution, increasing the resolution in Photoshop will not usually improve the image quality because in this case, Photoshop must in effect spread the same pixel information across a greater number of pixels.
The proper image resolution to use for an image depends on how you intend to display or distribute the image. Using too low a resolution for a printed image results in pixellation – large pixels that produce very coarse-looking output. Using too high a resolution (ie pixels smaller than an output device can reproduce) increases the file size unnecessarily and may increase the time required to print or distribute the image.
The pixel setting of the monitor along with the size of the monitor determines the size (and therefore density) of the monitor pixels. When converting printed images to on-screen images and translating image resolution into pixel dimensions, it’s useful to know that the default resolution of a Macintosh monitor is typically 72 dpi; the default resolution of a PC monitor is typically 96 dpi.
In Photoshop, image pixels are translated directly into monitor pixels. This means that when the resolution of an image is higher than the monitor resolution, the image appears larger on-screen than its specified dimensions. For example, when you display a 1×1 inch image with a resolution of 144 ppi on a 72-dpi monitor, it appears in a 2×2 inch area on-screen. Because the monitor can display only 72 pixels per inch, it needs two inches to display the 144 pixels that make up one edge of the image.
If you’re preparing images for print, it’s important to understand that printer resolution – that is, the number of dots per inch (dpi) – is usually proportional to, but not the same as, image resolution – that is, the number of pixels that make up an image and that determine the size of the image on-screen.
65lpi: Coarse screen commonly used to print newsletters and grocery coupons.
85 lpi: Average screen often used to print newspapers.
133 lpi: High-quality screen typically used to print four-color magazines.
177 lpi: Very fine screen typically used for annual reports and images in art books.
* 85 lpi: With coarse screens, resolutions at the low end of the range can produce good results.
* 177 lpi: With fine screens, only resolutions at the high end of the range produce good results.
Screen frequency and image resolution
Many commercial and desktop printers use halftone screens, which consist of printer dots called halftone cells, to print grayscale images and color separations. Screen frequency, also known as screen ruling, refers to the number of halftone cells per inch in a halftone screen, and is measured in lines per inch (Ipi).
The relationship between image resolution and screen frequency determines the quality of detail in the printed image. As a general rule, to produce a halftone image of the highest quality, use an image resolution that is 1.5 to 2 times the screen frequency. In some cases, however, depending on the image and the output device, using a lower resolution produces good results.
Example: for a screen frequency of 150 lpi set the image to 225-300 dots per inch.
About file size and resolution
The file size of a digital image is measured in Kilobytes (KB) or Megabytes (MB) and is proportional to the total number of pixels in the image. Although images with more pixels may produce more detail at a given size, they also result in larger file sizes. A 1×1 inch 200-ppi image contains four times as many pixels as a 1×1 inch 100-ppi image and so is four times as large.