M&G Electronic Prepress

Electronic Prepress:
Scanning

Resolution
The resolution of an image is expressed in terms of “dots per inch” (dpi) or “pixels per inch” (ppi). Line screen is expressed in terms of “lines per inch” (lpi). For grayscale or color art to reproduce correctly, the resolution should be approximately 1.5 to 2 times the line screen to be used at 100% of the placed size. This means it is very important to make proper calculations if you will be enlarging your graphic by a significant amount. For instance, if you intend to scan your graphic at 100% of it’s original size, but place it in your layout application at 150% of it’s original size, you should scan your graphic at 450 dpi when using 150 lpi for the best quality output. Below is a guideline for the required resolutions at 100% of their placed size:
• 150 lpi = 225-300 dpi (cover art)
• 133 lpi = 200-266 dpi (text art)

Scanning at a resolution over 2x will not improve image quality and in some cases will even degrade it. It will also create files much larger than needed which will slow production time. It is truly best to scan your image at the correct size and resolution that will be used. This will allow for easier management and storage of your files.

One-color line art–like a logo or pen-and-ink drawing–should be scanned at 600-1200 dpi at 100% of its final size.

Highlight and Shadow Values
The scanner operator or retouch artist should maintain the following values:
• Sheetfed press: 3% highlight, 90% shadow
• Small format web: 4% highlight, 88% shadow

(up to 6” x 9”)
• Large format web: 4% highlight, 85% shadow

(up to 8.5” x 11”)
• Cover halftones: 2% highlight, 94% shadow

Because of the need to compensate for dot gain, images will often look flat or washed out on screen. The best way to proof scan quality is with a press test; a high-resolution (1200 dpi or better) imagesetter proof is also good.

Never make adjustments based on screen appearance or laser prints–an image that looks great on screen will often lose detail in the highlights and shadows. Instead, please refer to the specifications provided above.