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The big picture of scanning-part #1
(reading time 4 minutes)
The murky details of bringing images into a computer seem to confound even the most experienced user. One barrier to the mastery of scanning is the widespread and inconsistent use of technology specific terminology (i.e. jargon*).
Once you understand the big picture though capturing images starts to make some sense and the decisions that you need to make when scanning or taking a digital picture become easier. If you want the image that you scan to be the proper size and sharpness you need to know how it is going to be used before you scan and you need to know something about the device where the scan will be displayed.
The short of it
All computers have two types of devices attached to them: input devices for acquiring information and output devices for presenting that information. In the era of graphic computing these input and output devices are graphic in nature. Examples of graphic input devices are a mouse, a scanner or a digital camera. Examples of graphic output devices are a printer, fax machine or your screen. You don't generally think of your screen as an output device but what it does is graphically display the content of your computer's memory and is really no different than your printer in that respect.
Each of these output devices has a maximum size in inches AND pixels, a defined pixel density in pixels per inch (ppi) and an ability to display a certain number of colors. While these values can be changed by altering settings in the computer or printer here is a chart defining some of the values for devices that you might create a scan for:
| Output device | Maximum size in inches | Maximum size in pixels | Pixel density in pixels per inch (ppi) | Image Type | | printer | 8.5 x 11 | 1275 x 1650 | 150 ppi | color | | fax machine | 8.5 x 11 | 1700 x 2200 | 200 ppi | black and white | | typical 15" screen (web, email, image editor) | 8.5 x 6.4 | 640 x480 | 75 ppi | color | | typical 17" screen (web, email, image editor) | 10.6 x 8 | 800x600 | 75 ppi | color |
The red numbers are calculated values based on known numbers1
Lets look at some specific scanner software and see how it relates to the output device that you are sending an image to. These software dialog boxes may not be exactly the same as your software but it should be similar. In the "easy" mode you see a limited set of choices
Each output device that you will be sending a scanned image to has a limited number of colors (sometimes referred to as color depth2) that it can display. A screen or printer can generally display 16 million colors while a fax can only display two colors (black and white). By choosing the image type you are telling the scanning software how many colors or gray levels your image has. This value is selected in the image type dialog box.
Each of the output devices also has an ideal pixel density. To achieve the best scan for any device, you need to know what that ideal pixel density is. With that knowledge you can create scans that are no bigger than they need to be, display correctly and are appropriately sharp for the way you intend them to be viewed.
When you are in the "easy" mode the scanner software assumes that you want to scan your image at 100% of its original size. It selects the entire photo or graphic. In this mode there is no control over size. You can not enlarge or reduce and must use the size of the original as your final sized image. Only by controlling this final variable can you truly optimize your scans.
While the "easy mode" doesn't require many choices, it has some drawbacks. Scaling or cropping of the image is one such limitation. The "easy mode" works fine as long as the photo you are scanning is exactly the size that you want. If your photo is too large or too small you can not adjust it and if you want just a part of the picture you have no ability to select it. The scanner software in this mode also makes the assumption that you know where you are sending the image and sets the pixel density appropriately. If you want some intermediate pixel density or to go out to a device not listed then it provides no options. Most importantly from my perspective is that since it masks all of the critical detail from view, it is also very difficult to learn much about scanning. To take the next step you need to move to the advanced scanning mode.
In a future article I will cover some of the advanced options- Scanning-The long of it.
footnotes
*In my opinon, if jargon were sin then computers would be Peyton Place and scanning software would be a veritable den of iniquity.
1.These are calculated numbers based on the known values. You know that paper is 8.5 x 11 inches and you know (although you may not know you know) that your monitor is 800x600 or 640x480. When you know the pixels per inch you can then calculate the other value with the formula "inches = pixels/ppi" or "pixels= inches x ppi" I know that algebra wasn't that much fun in 8th grade but did you think that your teacher was thinking of scanning when she said someday you'll regret not paying attention.
2.Why is it called color depth? The terms come from the fact that pixel colors are defined by the amount of computer memory that is allocated to the pixel. The more memory (the more bits), the more colors. The more bits the "deeper" the memory and the more colors that can be made available at that pixel. So it is referred to as 8 bit color depth (256 colors) or 24 bit color depth (16 million colors). For a more detailed explanation go tothe following web site:
http://fla.esf.edu/cavlab/Reference/candbd.htm
3. De-screen refers to the process of removing the pattern that is in professionally printed images in newspapers and magazines. Traditionally these images were photographed through a screen in preparation for printing. Thus they are "screened." If you don't "de-screen" it, it will have a pattern of rosettes that are called a moire pattern making the image look mottled.
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