Digital files are made-up of a grid of pixels. The question is, how many pixels wide by how many pixels high?
Scanners rate their ability to generate pixels in resolution. The "real" resolution of a scanner is the "Optical Resolution". Many scanner manufacturers also advertise a higher resolution known as an interpolated or "software" resolution. Quite simply, ignore that spec!
Let's take a look at a scanner with a 600x1200 optical resolution:
Scanners pass a CCD (imager) across the picture. The CCD of a 600x1200 scanner is 600 pixels (per inch) wide x 1 pixel high. The 1200 in the spec means that the scanner's motor that moves the CCD across the image goes slow enough to "stop" 1200 times per inch, thus generating 1200 pixels per inch along the moving axis. However, since the CCD is only 600 pixels per inch wide, the resulting image's width would have to be interpolated up to 1200 to match the capture along the moving axis. Therefore, the true "optical resolution" of the scanner is 600ppi, anything higher requires interpolation either in part (one axis) or as a whole (both axis's).
Using this 600x1200 scanner at 600ppi (its true optical resolution) would produce the following files:
4" high x 5" wide original:
8" high x 10" wide original:
Do you need this much resolution? That depends on what you are doing with the file. The output device (monitor, printer, etc.) determines the requirement of the scan resolution.
Scanning for Monitor Viewing:
If you were only going to view your images on a monitor, then the above scans would be overkill. Typically, a computer monitor's resolution is set at 600 pixels high x 800 pixels wide (800 x 600). Therefore, the 3000 x 2400 or the 6000 x 4800 scans (above) would be way too big for a common monitor.
The appropriate scan resolution for a 4" x 5" original that would result in a 600 x 750 image would be 150ppi (600 pixels divided by 4" = 150ppi). If the original was 8" x 10", the appropriate scan resolution that would result in a 600 x 750 image would be 75ppi (600 pixels divided by 8" = 75ppi).
Scanning for Printing:
If you want to print a scanned image, more resolution is typically needed. There are many kinds of printers with different resolution requirements, but we will look at an ink jet printer for now.
Ink jet printers spray three dots of colored ink (typically) to represent the full color of each pixel. Therefore, the resolution required is 1/3 of the printer's dpi (dots per inch) rating. If an ink jet printer were rated at 600dpi, you would need a scan of 200ppi to reproduce the original without enlargement.
If the original was 4" x 5" and you wanted to enlarge it to an 8" x 10" print (2X enlargement), you would need to double the scan resolution (from 200ppi to 400ppi) to allow for the enlargement.
You must also factor in more resolution if you intend to crop an image. Let's say that you wanted to crop the edges off of your 4" x 5" original to "zoom in" on the subject better. If you cropped an inch from each end of the 4" axis and 1 1/4" from each end of the 5" axis, you would be left with 50% of the original's size (2" x 2 1/2" instead of 4" x 5"). Even though you don't physically "cut" the picture, this is exactly what you are doing when you crop an image on the computer. With this example, it would be necessary to scan the 4" x 5" image at 400ppi since 50% will be "cut away". 400ppi will allow for the cropping the original scan and ultimately outputting a 4" x 5" print at 200ppi (the 600dpi printers required resolution).
Sending too much resolution to the printer is not advisable either. Doing so slows the printer down excessively and in many cases, reduces the quality of the print. Therefore if you scan an 8" x 10" original and you want to print it out at 4" x 5", you should scan the 8" x 10" at 100ppi. This would result in a 200ppi output at 4" x 5".