Scanning FAQ
(Frequently Asked Questions)

This will assist you in creating good quailty scaned images. Three types of scanners are on the Recitation Hall Computing Site. These scanners are discribed below, and for directions on how to use each scanner, click on the name of that scanner.

  • A Microtek Color Scanner- 11x17 scan bed with transparency top for 35mm slides and negatives.
  • Two Epson 1600 Scanners with transparency tops for 35mm slides and negatives.
While each scanner has different software drivers, the premise is the same behind each one. There are some definitions and processes you should keep in mind before you start scanning.

The most important rule to remember when scanning is to scan your file with the settings that are appropriate for the output. In other words, know how your final image will be used. Is it for a computer presentation, desktop printer, professional printer, the Web, or will your images be made into slides? All of these options require slightly different handeling.

Definitions: DPI, LPI, and Bit Depth

  • DPI-dots per inch (or pixels per inch), the number of physical dots associated with a file. Generally, the higher the dpi, the better the resolution. Higher dpi also increases file size. As a general rule, the dpi should be about twice the size of the lpi (Lines per inch) of the printer. The Site's color printers output is 300 dpi. The Line Screen Default (same as lpi) is 60. So, you need not scan your image higher than 150 dpi. The gray scale printers dpi run between 300 and 600, their Line Screen Default is 85. Therefor again 150 dpi is adequate for good output. Too many extra dots combined with line screens will cause your image to become very muddy.

  • LPI-lines per inch, the lpi is how close the lines are place on paper during printing. For instance newspapers use about 85 lpi, while glossy 4 color may use 133 lpi or more. This is important when considering the output of your file. To many lines per dot will give a solorized effect when printing a gray scale image to the b/w printers.

  • bit depth-This is the amount of information that each pixel (or dot) carries with it. For black and white images, there is only one bit of data asocated with that pixel, it is either on or off. In other words, it is either black or white. For 256 colors/grays images, there are 8 bits of data stored on each pixel. For 16 million color images, there are 24 bits of data stored in each pixel.
The mathematical formula is is the following:
Number of possible colors, grays = 2 nth power, where n is number of bit depth.
Like dpi, raising the bit depth also increases your file size, but gives you more colors, and better color gradations.

Numbers to Keep in Mind

OutPut Device DPI
Most desktop Monitors 72 Mac/ 96 Win. dpi
300 dpi Laser Printers
53 Line screen
100 dpi
600 dpi laser printers
85 Line screen
150 dpi
300-600 dpi laser printers
with the photogray option
106 Line screen
150 - 200 dpi
1440-2880 Photo ink jet printers
240-300 dpi
Saperations at 125 Line screen 200 dpi
Separations at 175 Line screen 300 dpi

Definitions: Scan Types

  • Scan types- are the different ways a scanner creates an image.

  • Line art-If scanning Gray Scale or Halftone images, this process converts the entire image to black or white. Any level of gray will be converted to black or to white.
    You would select this scan type if you were scanning hand drawn b/w illustrations, fonts, and old wood cut types of illustration from archival sources (such as Dover Books).

  • Halftone-simulates halftone screens printing screen. In older newsprint you could see small dots of varying sizes creating the illusion of gray scale. Comic books use larger screen dots. You can select different screen sizes and shapes (even lines) for various effects on your image.

  • Photo-Gray Scale and Continuous ToneAre all equivalent. This type of scan can (if selected) create the highest number of colors (16 million) or grays (256). Select this option when scanning color and b/w photos, watercolor, and color illustrations-any image that is a continuous tone. However, being the best has its costs. This scan type also generates the largest file sizes.

File Formats

The scanner software available for the Macintosh is capable of saving into many file types (including formats for Windows computers). The program in which your final image will used affects the file format you will save your file as.

Here are some general guidelines of the best file types for a particular program:

Program File Types
Photoshop PICT, TIFF
Illustrator PSD, TIFF, EPS, PICT

File Size

You should approximate the final output size of the image you are scanning. If you know you only need a 3x5" image, don't scan it at 8x10". Planning ahead will save disk space and processing time. Here are some general sizes for scanning an 8.5" x 11" picture with millions of colors:
DPI Approximate File Size
100 2.5 megabytes
300 22 megabytes
600 90 megabytes

Larger file sizes also mean longer processing and printing times. The printers at the Recitation Hall Computing Site are 600 dpi, so if you are printing in this site, or to a printer of similar quality elsewhere, don't scan your image higher than 150 or 200 dpi (unless printing separations).

Another way to reduce file size is to "crop" the image. This process involves scanning only the area of the image you would like to include in your final scan. If there is an object of interest in an image, cut out some of the background. The file size will be reduced because you are scanning less of the object.

The Theory of GIGO

GIGO stands for "Garbage In, Garbage Out." While scanners are impressive in their ability to digitize pictures for computer use, they are not magic. In fact, they only do what you tell them to. When you choose an image to scan, make sure it is of good quality. The image should have no dirt, scratches, or creases on it. While photo-retouching in Photoshop can remove these blemishes, it takes a skilled hand and a lot of time to perfect an image. Problems such as overexposure or underexposure make it even more difficult to produce acceptable output. In other words don't scan these types of images.

Unknown or Archival Use

When scanning images for a future unknown use, or for archival purposes, it is generally a good idea to scan the image at a larger size and at a higher resolution.  This way the image could be enlarged (a little) or reduced in the future to meet needs of out put at that time.

For instance, you may have an image that is 5x7 that you could enlarge to an 8x10 format when scanning.  You could also increase the scan resolution to 400 hundred dpi (ppi in some scanners).  This would give you plenty of pixels to move around for enlarging the image for printing, and still keep it from looking pixilated.