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| Nikon's Coolscan 9000 |
RESOLUTION
As with a digital camera, high resolution means big prints and large file sizes. Film scanner resolution is often expressed in linear dpi, or "dots per inch." Just remember that in this case a dot is the same thing as a pixel. A film scanner that provides 3,600 ppi true optical resolution, such as the Pacific Image 3650u ($269 street price), at full resolution produces image files that measure something in the neighborhood of 3400 x 5000 pixels. If you use a typical printer setting of 200 ppi you can make a print that measures 17 by 25 (or larger) with no problem. Big the files are huge -- around 52 MB each when saved as 8-bit TIFF files. That's more than 15 times larger than images that come out of an 8-megapixel Canon EOS Digital Rebel XT.
But hold the phone. You may not need all of that resolution -- it all depends on how you'll make use of the image. Unless you are making archival files or planning on producing 11x14 or larger prints you may be able to dial down the resolution a bit. You can scan at 1000 ppi and get images good enough to make 4x6 inch prints. If the images are for web use you can lower the resolution even more.
INTERPOLATION
Microprocessors can increase image resolution by inserting pixels in between those recorded by the sensor. The color of each new pixel is derived through analysis of the existing pixels located closest to it. Complex formulas, called algorithms, are strictly followed and the results are usually pretty good. But it's still guesswork. True optical resolution trumps interpolation every time.
DYNAMIC RANGE
Scanners with a high dynamic range are capable of producing images that simultaneously capture excellent highlight detail and excellent shadow detail. Unfortunately, some scanner manufacturers list the "computed" dynamic range instead of the actual measured range, just as some use interpolated resolution instead of optical. Pay attention to the measured dynamic range specifications and ignore the theoretical hype.
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