Halftones Vs Line Art Vs Continuous Tone Images Vs Graphically Reduced Images
What is a halftone?
Halftones are used to reproduce continuous tone images, such every bit photographs, on a printing press. A black and white photograph, for example, is actually made up of black, white and infinite shades of gray. A printing press applies ink one color at fourth dimension (per caput on the press), and as such, it is not feasible to apply every shade of greyness needed to create a realistic representation of the image. To compensate, printers have adult the process of halftoning where each individual photograph is reshot through a screen, resulting in an image composed of patterns of different sized dots that fox the eye into seeing continuous shades of gray:
Traditional halftoning techniques, however, are costly. Printers will charge nearly $eight-10 per paradigm to reshoot your photographs and create halftones. Through desktop publishing technology, however, anyone tin now reproduce photographic images digitally (through scanning), edit and raise the images (in Photoshop or whatever epitome editing app), and identify the images as digital halftones in desktop publishing programs such as QuarkXPress and InDesign.
What is image resolution?
One time you browse an image, it becomes digitized—made up of hundreds of thousands of pixels. Pixels are nothing more very tiny colored squares (there are 72 pixels in an inch) that y'all tin can run across if you increase the magnification of any image:
Resolution is the number of pixels in a linear inch (i.eastward. pixels per inch or ppi) but is more commonly referred to as dots per inch (dpi). The more pixels, or "dots," per inch, the college your image resolution will be.
With color images, each pixel can exist one of 16 million different colors (with a good scanner). For blackness and white images, in that location are 256 levels of gray pixels: 0 (black) through 255 (white). More pixels means higher resolution, which creates better paradigm quality because you stop upwardly with more than realistic representations of colour, ameliorate gradations of both individual colors and gray tones, and crisper images in general. Here is the same image at 72 dpi and 25 dpi:
The most of import thing to sympathise almost resolution is the relationship between an image'due south resolution (dpi), an image'southward file size (measured in kilobytes or megabytes) and an image's print size (width and peak). There is much more on this in the Photoshop tutorial, Image Size and Resolution.
What resolution should I use for my images?
This is a frustrating question because it all depends on your terminal print output, and more specifically, what line screen frequency, meliorate known as lpi, your commercial printer uses. Since nearly people option a printer afterward their project has been completed, this tin can exist tricky. Employ a resolution that is as well depression, and you end up with poor quality graphics. With a resolution that is also high, you end upwardly with longer procedure time and little gain on the overall quality of the halftone. There are, however, some typical line screen frequencies that are used equally general guidelines:
- Newspapers or publications printed on newspaper stock use a 65 or 85 lpi.
- Books printed on uncoated stock use a 120 or 133 lpi.
- Books and magazines printed on coated stock use a 133 or 150 lpi.
- Loftier quality books with lots of graphics (art books), calendars, etc. on coated stock will use 150 or higher lpi.
When determining resolution, the bones rule of thumb is that last prototype resolution should be twice that of your intended commercial printer's lpi. So, for example, if you are creating a family history book with black and white images, you tin't go wrong with an image resolution of 300 ppi. Withal, 267 ppi is the well-nigh pop ppi for most book projects with grayscale graphics.
This rule does not apply to line art. Line fine art (clipart, charts, etc.) is best handled as an EPS graphic with a resolution of at least 600 dpi, and 1200 dpi is all-time. You can make halftones out of EPS line fine art simply it really doesn't make much sense to practice so.
Which graphic file format is best for halftones?
Macintosh or PC/Windows, you can't become incorrect with TIFF format, and information technology is the preferred and the well-nigh widely supported image format.
What is dot proceeds?
Dot gain is when your halftone dots (pixels) swell as the ink is absorbed into the paper during the press process. It can misconstrue the final output of your images past making them appear darker and blurry. Dot gain can occur in a number of ways—printing printing miscalibrations, poor newspaper quality and the general limitations of the printing process itself. At that place's very trivial you can practise to avoid dot proceeds, just there's quite flake your printer can do at press time to compensate for dot gain. Almost of the time, it is best to let the printer make the necessary adjustments to your page layout files.
When editing photos in Photoshop, keep in mind that midtones create the highest levels of dot gain, so if your midtones look also dark when y'all test impress images from a laser printer, chances are that they will be fifty-fifty darker when they are printed on a press. The higher the lpi, the more than dot proceeds. Newsprint and porous (rather than smooth) uncoated paper stock produce the most dot proceeds.
Source: http://www.espressographics.com/text/halftoning