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Better Scanning Techniques

How to squeeze the utmost quality out of your film/scanner combination.


By Tom Ang

Scanning silver-gelatin negatives
The scanning of normal black-and-white negatives (silver-gelatin images) presents special problems. If you have scanned black-and-white negatives at high resolution-say at least 2700 ppi-you probably noticed that the results were grainer than you expected. Or at any rate, they were much grainier than a scan from comparable color material. And if you tried to scan at even higher resolution, the problem became worse.

Fundamentally, the difference is that the silver particles making up the image scatter light, whereas the dye-clouds of color images absorb and filter light. At the same time, the lighting in a film scanner is highly directional; it is hard lighting. The net result is similar to printing with a condenser enlarger: Darkroom workers know that printed results display sharper-edged grain and higher contrast than those from a diffused light-source enlarger. Now, add this to the high-resolution raster of a scan and you create interference or aliasing between the film grain and the regular array of pixels. The resulting scan is artifacted.

Imagine a sharply defined grain: If it lies wholly within a pixel, it is accurately recorded. But if the grain only covers one pixel and half of another, it will register on both pixels, fi ling both. Thus, it appears to be twice its actual size. The result of grain/raster interference is that many silver grains appear larger than they really are. Similar problems can be encountered with color film, but they are, in my experience, not as marked as with black-and-white film.

You can try to reduce grain/raster interference through simple light defocusing-the classic botch for low-pass filtering. Set the scanner driver to manual focus if possible, or fool it by raising the film with a clear piece of film. This stops very fine detail from getting through. While this seems to run counter to common sense, it may reduce graininess without harming broad image features.

Another way to reduce the problem is to use chromogenic black-and-white films such as Kodak CN400 or Ilford XP2. These films create the image using dye clouds rather than particulate silver.


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