Choosing and Using a PC Scanner
Just the Basics
A 1200x1200 optical dpi scanner is recommended for general use; a 2400x2400 or better optical dpi scanner is recommended for film negative and slide scanning. Mine is a 600x600 dpi 30 bit color model from several years ago.
I will probably scan at 300 dpi most often but once in awhile I may want to scan small super detailed objects like postage stamps. I will usually select 75 or 100 dpi for pictures to go onto web pages.
Picture detail versus size of picture on the screen is the same and is a tradeoff regardless of scanner model.
Don't buy a lower resolution scanner just to match your printer. You can always choose lower resolutions to scan at. Or you can print the picture twice the size to see all the detail.
The higher the resolution you scanned at, the more space on the hard drive the picture consumes. The more colors you scanned at, the more space on the hard drive the picture consumes.
Try to keep resizing pictures to a minimum. Don't resize a picture more than once.
Plan on playing and experimenting with it awhile before doing serious work.
And Now the Details
We suggest at least 1200x1200 (1200 dpi both horizontally and vertically) as optical resolution, even if your printer has less resolution. Scanners that handle slides or negatives should have at least 2400x2400 dpi optical resolution. I would go for 30 bit color which provides 10 bits of color for each of red, green, and blue. The older standard was 24 bit color.
I prefer the flatbed style, so I am not restricted to sheets of paper or thin cardboard originals. The feed-through style takes up less desk space and is more likely to be bought for use with a laptop. Also I plan to scan player piano rolls, 11 or so inches' worth at a time, which only a flatbed will accommodate.
The number of bits determines how many color shades can be represented. A 24 bit scanner has 8 bits for red, 8 bits for green, and 8 bits for blue for a total of 256 gradations of each color's contribution to each dot in the picture, or a total of 16 million possible colors. You will be able to tell the difference between a picture with 24 bits of color and a color snapshot but it won't be objectionable. With 30 bits of color (and a professional grade color printer) it will be hard to tell the difference from a color snapshot. With an average ink jet printer, even a 1440 dpi model, you can still tell the difference.
Most scanners nowadays have a USB (Universal Serial Bus) connection. A few have parallel port connections. These do not require opening up the computer. The SCSI interface is still seen from time to time and usually requires installing a board inside the computer.
I admit that most of the time I will be scanning things as 300 dpi or lower. The 600 dpi scanning is needed only for such things as postage stamps and maps from your AAA or Rand McNally road atlas. But it might come in handy some day.
A typical 4 by 6 snapshot made from a decent 35mm camera needs 300 dpi to preserve all the detail. Scanning at 200 dpi may suffice for a Polaroid snapshot or one from a disposable camera. A newspaper photograph has about 85 dpi of resolution but sometimes needs up to 150 dpi scanning resolution to eliminate moire patterns.
Things scanned in at 600 dpi, and even things scanned in at 300 dpi can only be seen in their full glory on a computer screen when shown magnified.
All PC's nowadays have a screen resolution called VGA of 640 dots (pixels) screenwide by 480 dots high which is roughly 75 dpi on an average sized monitor. Or something similar. Most have Super VGA 800 by 600 (95 dpi). Many have XGA which is 1024 by 768 (120 dpi). Some have 1280 by 1024 (150 dpi). A few have 1600 x 1200 but only with an above average sized monitor. Resolution is limited to these choices. You can change back and forth as often as you want. A physically bigger monitor does not give more resolution unless both it and your computer's video card offer more of the preceding choices when used together.
After the screen resolution is chosen, the more picture detail you want the more screen space the scanned in picture will occupy. For creating web pages, you first decide how you want to trade off picture detail versus space consumed. Then you choose the scanning resolution that will give you what you decided on. Or resize the picture after scanning.
If you scan in a postage stamp (about an inch square) so it displays on your screen actual size, it is only going to occupy a space 75 dots by 75 dots more or less. It will have most of the details blurred out.
Or you could scan the stamp in at 600 dpi and then display it normally. It comes out magnified to fill the screen (600 dots wide) and you see all of the detail. I believe that some detail will not be picked up when it is scanned at 300 dpi.
If you are scanning in pictures expressly for viewing on the screen, or putting on web pages, you will usually scan them in at 50 to 100 dpi. A web page should be designed to look good at 600 dots wide, because lots of computer owners run their monitors at 640 by 480 dots resolution. Pictures scanned in at 50 dpi will look a bit smaller than the original, pictures scanned in at 75 dpi will look about the same size, and pictures scanned in at 100 dpi will be magnified a little on the computer screen.
The higher the resolution you scan the picture at, the more space on your hard drive it will consume. Therefore in general you should scan pictures at a higher resolution only if they are actually going to be viewed or printed using that resolution with or without magnification. There are some exceptions that are too complex to describe here other than what you may run into by trial and error. But I think it is never desirable to scan at more than twice the resolution that the picture will ever be viewed or printed with.
Nothing wrong with scanning in a snapshot at 300 or 600 dpi, then cutting out a small part to put on a web page, then deleting the rest of the file that could be over 20 megabytes in size. Better yet, cut a rectangular hole in a piece of white paper, align the snapshot with the desired part over the hole, and then scan it.
Also, if you have scanner with 30 bit color, some of the time you may be selecting 24 bit mode. Or maybe even less. The more bits of color, the more space on your hard drive the picture will consume.
Scanning an original again at a different resolution to achieve a different size on the screen usually gives better results than resizing an already scanned picture. I can safely say never resize a scanned or bitmap picture more than once. Every time a picture is resized, the computer has to either discard or interpolate (generate or regenerate) information. Even if you resize a picture to exactly the size it was originally, the quality gets worse, not better.
Sometimes if the scanned picture is too dark or too light you will be able to use the software to adjust it after scanning. Sometimes you will have to scan it again but that is usually not too difficult, only time consuming.
Much more and also easy to understand material on scanning is on this web site by Wayne Fulton:
http://www.cyberramp.net/~fulton/scans.html
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