Scanning

Scanning

THE DIGITAL CORNER

This will be a regular feature of the Photogenesis Newsletter covering technical, creative, and other issues for the digital photographer.

The topic this month is scanning: A comparison of scanning prints versus film. Flatbed scanners are now fairly common and inexpensive, so many people can now scan flat, reflective artwork. Relatively few flatbed scanners have "transparency adapters" [more available now, but still more expensive] to allow scanning film, and fewer still are good enough in spatial resolution, bit depth, and noise level, to actually be useful for scanning film. But there are some real trade-offs between this type of scanner and a "real" film scanner, the biggest one being cost. Flatbed color scanners range in price from almost free to a few thousand dollars. Most film scanners available to consumers are limited to 35mm film and cost from about $500 to $3,000. Scanners for medium and large format films generally cost much more, so I'll limit this discussion to 35mm format with regard to film scanners.

A big part of the question is whether you use negative or reversal films. Color negatives are very non-linear and require a larger dynamic range, as well as some patience and skill, to produce good scans. For this reason, a mid-range flatbed scanner used on an 8x10 print will most often yield a better image than a low-cost film scanner of about the same cost. A large print is important because flatbed scanners usually have a resolution of only 300 to 600 dots per inch [1200 dpi or higher is common now, but there isn't really that much detail in most prints, so the argument still holds], while 35mm film scanners are usually 2000 to 3000 DPI. (Remember that scanner manufacturers often claim much higher resolutions via "interpolation", but this is nonsense. Ignore such numbers and look for the actual optical resolution).

Prints have a much lower dynamic range than film. In fact, the 24-bit color provided by most flatbed color scanners is more than enough for scanning prints (if other quality factors are met). However, a film scanner (even those limited to 24 bits) can get more useful information out of a slide because the exposure can be varied to "extract" those 24 bits from different parts of the dynamic range. So if you are working with a print that does not have the tonal quality you want you will be much happier to work with a slide in a film scanner.

Both types of scanners are based on CCD arrays [some are now CIS type, with somewhat different characteristics], which usually have a problem with noise in dark regions. There's no reason to think that this problem would be worse in one type of scanner versus the other, but it definitely does vary from one model to another, and is generally better in the more expensive versions of either type.

Film scanners exhibit another interesting weakness: Just as a condensing lens enlarger tends to show scratches in the film more than a diffusing enlarger, film scanners using condensers tend to show scratches and dust more than you would see on a print.

In summary:

- Flatbed scanners are cheaper and more versatile, but give poorer

results in most cases, and are quite useless for scanning small

images (such as 35mm film) or any size color negative.

- Scanning film gives you a lot more control, assuming that you

can't control making a print and then scanning that.

- Color negatives are difficult to scan in any case, and require a

better dynamic range, perhaps 36 bits, which is equivalent to

a density range of 3.6.

- If you're interested in digital restoration of old photos consider

whether film will be available to you. Old negatives are far more

likely to be scratched, bent, or unavailable than prints.

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