Flatbed scanners digitize your old slides and negatives

Berti Kolbow, Deutsche Presse-Agentur, Bochum, Germany

A scanner may well be the second most important peripheral on your desk.

Today's scanners can convert pages of books into editable text, make copies, read and convert business letters, and turn photographic prints into bits and bytes ready for Photoshop.

They've certainly come a long way since the Stone Age of computer development, when cutting-edge scanners were handheld devices that read lines of newspaper text with more errors than hits.

Today's newest units have added a feature that's tempting more people to upgrade their existing scanners or buy new ones: the capability to turn old slides and negatives into digital images, with the help of included transparency units. But not all scanners handle slides and negatives with equal aplomb.

Scanners that work with slides and negatives can cost hundreds of dollars, significantly more than the cost of traditional flatbeds today.

So it's important to carefully consider several factors for handing over your money, says German consumer expert Jaroslav Smycek.

If the unit is to bring slides or negatives into a format in which they can later be enlarged with all of their colors and gray levels intact, it is essential to judge a scanner's power by its resolution and optical density.

"The density indicates how well a scanner can depict the dark points of an image," explains Peter Nonhoff-Arps, editor of c't computer magazine.

The contrast range - the difference in lightness between the lightest and darkest parts of an image - is much larger for slides than for photo prints.

This means that a scanner's maximum density should be as close to possible to the theoretical maximum density for slides, which is "4." Otherwise, the darker areas will be unsatisfactory, with little or no definition in shaded areas.

For scanning typical photographic prints on the other hand, a maximum density of 2 is sufficient.

Scanner resolution is indicated in dpi, which stands for "dots per inch."

This is the measurement of the maximum detail that a scanner can pull out anything placed on its bed. 1000 dpi, for example, means that a scanner can read a one inch line so precisely that exactly 1000 pixels will be used to depict it on the monitor. For slides, negatives, and photos, more resolution is typically better.

But users shouldn't be fooled by promises on the packing like "4800 dpi!"

"In reality, only half or two thirds of the nominal resolution is achieved," says Nonhoff-Arps. He tested both high and low end flatbed scanners early this year for c't.

According to his tests, devices that claim to achieve 4800 dpi actually average scans of only 1700 dpi when running in backlit mode. This is sufficient to view a slide on a PC or to print it out in 13 x 18 format at 300 dpi, but not enough for a DIN A4 sized poster.

For text recognition and for photos that are scanned photographically, an actual resolution of 300 dpi is sufficient. More affordable scanners available between 100 and 150 dollars with a nominal resolution of 1200 dpi can handle this without problem.

The manufacturers' indications of resolution are a reflection of the strength of the so-called CCD sensor, explains Dieter Roether from Canon.

An optical system transmits an image of the photo, slide, negative, or document on the scanner's bed to the sensor, where the information s digitalized.

The highest theoretical resolution capable from the optical system and the CCD is actually never achieved because lenses and glass plates are situated in the middle of the system. These lenses and plates inevitably lower the maximum possible performance.

But special slide and negative scanners place less glass between the original and the sensor, by contrast. The actual achieved resolution for these devices is therefore much higher.

If the main function of your scanner is to be the occasional copying of pages and sending off a fax, then a simple device costing no more than 150 dollars should suffice, Smycek explains.

Remember that even the most pedestrian of flatbed scanners these days can do a decent job of scanning photographs. But if slides and negatives are going to be your stock in trade, then a unit specifically designed for this purpose makes sense.