Despite the rise of digital cameras, people are still needing to scan images onto their PCs. Scanning the image will enable it to be sent to other people, digitally restored or preserved to prevent further degradation. If you can get the scanning resolution right, then this will make further manipulation of the image much easier.
When choosing the best resolution, it can be a little tricky trying to find the right balance between file size, scanner resolution and current system resources. These choices are made more complicated by two misconceptions - the first is resolution of the scanner.
Most scanners ship with two types of resolution: optical and software. The optical resolution is based on the mechanics of your scanner. The more precise the components, the greater the resolution that a scanner can achieve. Most new scanners have an optical resolution of 600x600dpi (dots per inch) or higher.
Even though a scanner may have the mechanics to scan at 600x600dpi, it is not unusual for it to be promoted as something similar to 2720x2720dpi. This can confuse people, as this 'improved' resolution is performed by the scanner's software and has no direct connection with the mechanics of the system (it is also called 'interpolation'). The bottom line is that interpolation rarely offers any practical improvement to an image. It can also generate ridiculously large files and it is doubtful that any printer could print a file at a resolution of 2720dpi (in most cases, the extra data that was created by the scanner software would be discarded by the printer and the same results could be achieved with a much lower resolution image). Also consider that a single 8x10in image at 2720dpi would bloat to about 1.5GB (yes, this is 1500MB).
Now it's time for the confusing aspect of printer resolution versus scanner resolution. These two values are not equivalent - having a 1200dpi printer does not mean your scanned images need to be 1200dpi. In fact, most image files only need to be half the resolution (or less) of the printer. So, if your printer can output 1200dpi, then your file only needs to be scanned at a maximum of 600dpi.
This sounds confusing and counter intuitive to most people. Why does the printer need to have a higher resolution than the file it is printing? In very simple terms, a printer has to use a group of smaller dots to accurately recreate one dot from a graphics file - to print these smaller dots will mean that the printer resolution must be higher.
The next point to consider is what you will do with the image. If you intend to manipulate a photograph once it is scanned, then a higher resolution is more desirable. Once you are finished editing the file, then you can decrease the resolution ready for print.
When processing a large collection of photographs, you may need to consider the time factor for scanning, loading and saving each picture. Large, high-resolution pictures can take longer to manipulate. Finally, don't overlook the type of scanner connection to your PC. Parallel port scanners will take an eternity to scan a large image, but a scanner with USB or SCSI card will make the process much quicker.
WHICH RESOLUTION TO USE
If this discussion hasn't made you want to chuck out your scanner, then here is a rough guide to resolutions. Assuming you have at least a 200MHz PC with 64MB of RAM, then scan your images at 600dpi (if the maximum optical resolution of your scanner is less than 600dpi, then use its maximum setting). When scanning pictures above 8x10in, drop the resolution to 300-450dpi. After editing the image, you can reduce the resolution to around 300dpi. It is also a good idea to send test images to the printer, to see if your desired resolution will provide acceptable results. Saving the final image as a JPEG is also a compact way of storing images (do not use the GIF format, as this ruins the quality of most photographs).