Tutorial - Basic Principles of Scanning

TUTORIAL

Basic Principles of Scanning

(excerpt from the book Stephen Johnson on Digital Photography unreleased revised electronic version)

The following are some fundamental parameters to keep in mind as you are scanning your film. Remember, time and care invested up front not only protects your film, but results in a far more useful result. It is well worth the effort and time to make the best scan possible.

Everything is in the scan.

Straighten your original before scanning. Post-scan rotations other than 90° or 180° are time consuming and force image resampling (interpolation).

Dust the print or film (and the scanning glass on flatbed) before scanning. With film, extremely careful handling is very important.

Handle film with care. The original film is very precious, and it should be handled only by the edges and cleaned with film cleaner only if absolutely necessary. If canned air is used, be careful not to tilt or shake the can when spraying, as it can send out propellant and will fog your film. If there is something really nasty on the film that requires vigorous cleaning or rewashing, make the best scan you can first, then clean and rescan. This is a precaution just in case damage occurs from cleaning.

Warm up the machine. Most scanners need some warm-up time before the lamp stabilizes and the calibration procedure is accurate.

Make the best scan possible. Post-scan editing deteriorates the image (causing potential posterization) by stretching 8-bit 256 gray levels per channel to new values without necessarily filling in the gaps left behind (see, for example, the histogram below). If your scanner allows pre-scan color correction, use it to adjust your image to your needs. When necessary, it is better to make several scans to get the best scan possible than to heavily edit an 8-bit image after scanning.

Make the judicious pre-scan adjustments. Make sure your pre-scan adjustments aren't post-scan edits. Some scanning software can give you the impression that it is altering the way a scan is acquired, while, in reality, it is simply post-scan editing the image before allowing you to see it. Such adjustments are not usually effective, primarily because you could probably do a better job editing the image in Photoshop. Pre-scan adjustments are about controlling the conversion of the scanner’s data capabilities into a deep and editable archive file. This adjustment is usually accomplished in the A/D converter behind the CCD array, where analog data from the array is converted to digital data for the computer. It is much like adjusting the development of film to specific lighting contrast conditions of the scene. In high-bit depth scanners, this can essentially pull out of the scanner’s deep vision a result that may be almost finished.

Treat the scan as an archive. I recommend thinking of scanning as the creation of an archive of the image, capturing and holding the most information possible. Scan at 16 bits, use the highest optical resolution of the scanner, and be careful to preserve highlight and shadow detail. This can then function as your digital negative, from which all other edits and customizations will flow. Some photographers prefer to scan linear data preserving everything the scanner could in a somewhat raw form, skipping pre-scans adjustments, and doing all edits in Photoshop. As these files come in very dark, they can be a challenge to edit in Photoshop.

SilverFast, a great mulit-scanner Scanning software

Scan to the needed size and resolution. Acquiring more data than you need creates storage problems and slows down your computer. Acquiring less data than you need risks revealing pixels in your final halftone-screened image. Generally, you should choose 300 ppi as an accepted and mostly acceptable input ppi at your desired size. For 35mm film with its 1-inch height, that would take a 3000 ppi film scanning resolution to make a 300 ppi output scan at 8x10.

For halftone prepress needs, twice the ppi (1.5 works well) for whatever lpi halftone screen you intend to print. Watch out for scanning software that interpolates data. If your scanning software allows you to scan at a higher ppi than the scanner can see, it is creating—not imaging—those pixels. A scanner whose CCD is 400 ppi cannot acquire an image at 800 ppi without making up some information (such a scanner might scan 800 times an inch and only make up data in one axis). In general, scan at no higher resolution than the true optical resolution of the scanner.

Everything is in the scan.