Scanning Step by Step

If you've ever decided to scan an image, but you froze when you got to the scanner, then it's not the computer that has the problem, it's you. It's important that you think about what you're doing before you scan the image, as these initial decisions may be critical in the longrun.

You will go through a process of answering questions presented in the form of the dialog box in the scanning software. Your response is required in several areas:

1. Reset software defaults
2. Image type
3. Resolution
4. Scaling Factor

1. Line Art-a line art scan will be encoded in only two values--black and white, creating a bitmap image in Photoshop's image mode. As the CCD measures the values of each pixel, it uses a "threshold" and encodes readings under 50 percent as white, and over 50 percent as black. Examples of line art are pen and ink drawings and uncolored pages in a coloring book.
2. Grayscale-A grayscale scan is used for digitizing black and white photographs or black and white drawings that consist of subtle variations of gray tones, creating an 8,10, or 12 bit image. Images scanned in grayscale are made up of 256 shades of gray which are encoded in the black channel with values which range between 0 (pure black) and 255 (pure white).
3. Color-when scanning color images, use the color (sometimes called millions of colors) mode. Somes scanners make three passes, red, green and blue; others make a single pass using color filters. The bit depth of the scan (8, 10, or 12) multiplied by three, for the three color channels determines the scanner's bit depth. While some scanners boast 36 bits, Photoshop, the premiere image editing program uses only 24 bits of data.
4. Halftone-Don't confuse this image type with grayscale. Using the "halftone" type scan actually creates a bitmap image which has a bit depth of one and can be used to create special effects screens and simulated halftones. It might typically be used by someone printing to a dot matrix or non postscript printer.

This is probably the biggest stumbling block for those who are learning to scan. I've found that there is a lot of bad advice being propsed by those who are supposed to be in the know, such as, "Just scan as high as your scanner will go. Then you'll be sure to have enough resolution."

While this technique is sure to supply more than enough resolution, it is likely to cause additional problems on a number of fronts. It is likely that you'll end up with a file that is not only taking up much too much disk space, but that taxes the RAM memory when making editing changes in Photoshop and exhausts the memory of the printer, resulting in freezes or printing errors.

In order to determine scanning resolution, we only need keep in mind two very simple formulas--one for line art, which includes those special effects halftone images, and one for continuous tone images, which includes both color and grayscale.

Determine the resolution of the final output device and use that for your scanning resolution, remebering that the unaided eye has difficulty seeing resolution beyond 800 dpi. For most purposes, 300 dpi produces more than adequate resolution.

If you plan on enlarging or reducing your image, but still want to scan it in at actual size, instead of scaling it in the scanning software, account for the anticipated change by multiplying the output device resolution by the anticpated percentage of enlargement or reduction.

Example: If the output device resolution is 72 (monitor) and you anticipate enlarging the image 1-1/2 times, or 150 percent, your formula would read, 72 x 1.5 = 108, with 108 being the correct scanning resolution.

Don't use the above formula if you intend to use the scaling slider within the scanning software.

Determining the scanning resolution for grayscale or color images is just a bit more complex than the method for line art. Instead of using the ³resolution² of the output device, we must know the halftone line screen that will be used in printing and then capture two pixels of resolution for every dot in the halftone linescreen.

The halftone linescreen is generally based on a balance between budget and quality. The best quality is achieved by using a halftone linescreen of 150 or 175 lpi, but to do so makes an expensive job. Not all jobs require that quality. If you plan on printing camera-ready art from a desktop printer, then you need to know the printer resolution.

A 300 dpi laser printer can produce a 53 lpi linescreen. Anything greater will result in loss of gray values, or posterization.

A 600 dpi laser printer does a 65 lpi linescreen.

A 1200 dpi laser does a decent 85 lpi linescreen.

Ink jet printers, even at 1440 dpi, only produce the equivalent of a 65 lpi linescreen.

If your job requires a higher (more densely packed and smaller dots) linescreen, then you should consider getting your final output from a service provider where output devices can produce linescreens of 133, 150, 175 or even 200 lpi. The best way to determine what you need and what can be printed is to ask your printer what they require.

I recommend that you don't use any of the other features in the scanning software, such as curves, screen removal, sharpening, contrast adjustments, etc. Use the features in Adobe Photoshop to make those adjustments.

• Levels
• Retouch
• Despeckle
• Unsharp Mask