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What Is "Resolution" and How Do I Use It?

Bitmap images are composed of pixels. Image "resolution" is simply the number of "Pixels Per Inch" (PPI) in the bitmap grid. There are two aspects to every bitmap image - its size (width and height in inches) and resolution (the number of pixels per inch). These two factors alone determine the total number of pixels in an image. For example, a 2 inch by 3 inch image with a resolution of 300 pixels per inch contains (2 x 300) x (3 x 300) or 540,000 pixels.

The more pixels there are in an image, the more detail the image can be displayed with. The fewer pixels there are in an image, the less detail the image can be displayed with. There are two ways to display an image - on screen and in print. When you are preparing images you need to know what resolution to scan or size them to. There is an optimum resolution for each and it is very easy to determine.

First let us take up screen display. This is the easiest to determine. Scan or size them all to 72 PPI. Why? A monitor on a Macintosh system displays at 72 PPI. A monitor on a Windows system displays at 96 PPI. Though there is a difference between the two, the standard for screen displays is 72 PPI. Piece of cake!

Continuous Tones

Determining image resolution for printed output is a little bit different because of the way an image gets printed. A typical photograph like a snapshot or 8 x 10 glossy of your favorite movie star is known as a "continuous tone" photo. It is called a continuous tone because of the gradual changes of tones of color or shades of gray (for black and white photos).

Halftones

It is not practical to print continuous tones on a printing press so a method was developed to simulate the changes in tone using only black ink for black and white photos. For a photo to be printed on a printing press it must first be converted into a "halftone". A halftone is an image whose continuous tones have been converted to a pattern of solid dots. When viewed as a whole, this pattern of dots appears as a continuous tone, when, actually, it is not.

		This grayscale image when printed is really composed of a pattern of dots.

Using a camera and film, you could take a continuous tone positive and expose a negative with an ordinary window screen positioned between the lens and the film and produce a halftone. The light passing through the screen will diffuse and create the halftone dots. Professional litho houses and printers have been using a method similar to this, although not with a crude window screen. With computer graphics it is done by the software and the printer. You can create your own halftones on a laser printer to get a feel for this.

A halftone is expressed in terms of the "screen frequency" - the number of "Lines Per Inch" (LPI) of the screen used to prepare it. The number of lines per inch is also referred to as the "Line Screen" (LS) of the halftone. Halftones appearing in a typical newspaper range from 65 to 85 line screen. These are coarse halftones because the paper is very porous and rough and requires a larger dot size than the smooth, coated stock used in magazines or brochures. Halftones appearing in magazines, brochures and high quality catalogs typically vary between 133 and 150 line screen.

When preparing images for printed output you must know the screen frequency of the finished halftones before scanning or sizing them. Find out from the printer what is required for the job. There are a lot of variables you need to nail down ahead of time such as the type of paper, the capability of the printing press, what kind of printing plate will be used (paper or metal) and whether the plate will be exposed from a shooting board positive or a film negative.

Once you know the screen frequency of the finished halftone, you can scan and otherwise prepare your images. There is a rule of thumb used in preparing images for printed output. Scan or size images at a resolution (PPI) of 1.5 to 2 times the screen frequency (LPI) of the finished halftone. For example, a 2 inch by 3 inch photo would be scanned or sized to a 2 inch by 3 inch image at 225 to 300 PPI for a finished halftone at 150 LPI (150 line screen). Were the same photo to be printed using a 120 line screen halftone, it would be scanned or sized to 2 inches by 3 inches at 180 to 240 PPI. This is a very workable rule and is easy to remember.

You Determine The Halftone Frequency In The Layout

Once you have prepared your images, you import them into the page layout program such as QuarkXPress™, Adobe® PageMaker®, Corel VENTURA®, Microsoft® Publisher or whatever you are using. In the picture boxes or print dialog you fill in the data fields telling the software what screen frequency to print the halftones. The laser printer or high-end imagesetter uses this information at output time and produces the halftones on paper or film from the digital data. You use grayscale images for black and white halftones and CMYK (not RGB) color images for color separations.

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