By Lou Dolinar
Second in a series
Updated Feb. 14, 2006

Operating a digital scanner isn't rocket science, but there are still some basics you need to know, despite the best efforts of software designers to make these devices idiot-proof.

You typically get at least two pieces of software with a scanner that are relevant to preserving your photo memories: the driver and the operating software, and a formal retouching program.

The first sets the parameters of the scan and can optionally make corrections to faulty originals. These corrections include automatically removing dust and scratches, color restoration and grain reduction. You also can crop the scanned area and adjust light levels by hand.

Retouching software, meanwhile, loads separately and usually includes and extends the functions of the basic scanner software, giving you greater control over the finished scan. The manufacturer may throw in other products: for example, business card scanning software and OCR (optical character recognition) to turn scanned letters into type that can be edited.

Beginners should let the basic scanner software handle most of the photo fix-up work. As you get more comfortable with the process, switch off these features on the scanner software and try doing the work yourself in the retouching program.

Or, you can just archive photos as they come out of the scanner and rework them as your skills grow. However, there are four areas where you have to make judgment calls immediately in an archiving project, namely on resolution, color depth, compression (if any) and cropping.

Measured in ppi (pixels per inch), sometimes called dpi, resolution determines how much information you're capturing in a scan, which in turn determines how big an enlargement can be made from the original. Color depth is the number of colors captured, and we'll just specify here you want millions with a 24-bit setting. Compression determines how big that file will be on disk. Cropping, meanwhile, determines how much of the image you're going to save, and what you're going to discard, if anything.

Resolution. I'm going to grossly oversimplify the resolution issue; for a detailed explanation, the best resource I can give you is author Wayne Fulton's excellent Web site, www.scantips.com. The entire site repays careful reading.

Scanning software allows you to back into the ideal resolution for saving your photos to disk. In general, you want to set the size of the biggest document you'll ever need to print or display on screen - for example, 640 by 480 pixels for computer screens; 4 by 6 inches, 5 by 7, 8 by 11 for prints. You then determine the resolution you want for the final document. For virtually all home color printers, scan output should be set between 240 and 360 dots per inch at whatever sized print you're attempting. (Don't pay attention to the rated resolution of your printer; that's a different matter).

Color depth. The software then calculates input, such as how many pixels it needs to pick up from the original slide. In our hypothetical conversion of a color slide to a 240 dpi, 8-by-11 glossy for a color printer, we get a scan that's 2400 by 1920 pixels (4.5 megapixels), with a file size of 13.18 megabytes in the standard uncompressed format, TIFF. For a nice poster-size 11-by-17 print at 240 dpi, we need 4080 by 2640 pixels, (10.7 megapixels or 30.81 megabytes).

Note that we're talking about negatives and slides; your 4-by-6 prints are less useful, and generally can't be scanned and enlarged by much - an 8-by-11 enlargement of a 4-by-6 snapshot, printed at 240 dpi, is pushing the envelope. Snapshots still look fine when displayed on a TV or computer screen, however, which in most cases max out at 1280 by 1024 pixels. For bigger work, however, you'd be advised to go back to the original negatives.

A couple of observations here: If you already own a consumer grade digital camera (3 to 5 megapixels), you can see that a good scanner, when it's fed positive or negative film, easily outputs higher density files capable of greater enlargement. In fact, to capture all the information on a 35mm slide, you'd need up to 20 megapixels, depending on the type of film used and the quality of the slide, though 10 to 15 MP would be more typical.

Compression. This is where we get into some interesting trade- offs between disk space and scan quality. Assuming you may want to print out your photos at 11 by 17 some day, you're only going to fit about 20 of them on a regular 640 megabyte CD. Opt for a recordable DVD, on the other hand, and you'll have room for several hundred in 8.4 gigabytes.

Any way out of the space demands? Just as you can freeze-dry your CDs with MP3 compression, so can you shrink your scans with JPEG compression that usually incorporated into scanning software. The highest possible quality setting usually yields a file about 25 percent the size of the original; as with MP3s, you can opt for higher compression and lower quality.

Trouble is, even the best compression settings unavoidably damage your photos, and repeated editing and saving of JPEG amplifies the effect. So there's a pretty good case for keeping archival material in uncompressed format, but JPEG is great if you want to put a couple of hundred slides on a CD to hand out or use in a slide show.