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Better Scanning Techniques

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

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By Tom Ang

A number of film scanners use infrared light to detect dust specks prior to a mask-based removal of them. But silver grains scatter infrared very well, so dust removal based on infrared cannot work with black-and-white film, apart from those developed in a C41 process (such as Ilford XP2 or Kodak T400CN). Some systems will not work with Kodak Kodachrome film.

Input resolution
Poor numeracy skills combined with a confusing subject make resolution a topic that will baffle digital photographers for many years to come. One reason for this is that the term "resolution" is now used in at least five distinct senses:

* Input resolution: in points per inch (ppi), measures the fineness of details scanned-the frequency or density of data sampling.

* Output resolution: in pixel per inch (also ppi), measures the size of pixels in the output-the spacing of output pixels.

* Device resolution: in dots per inch (dpi), measures the fineness of output from a device such as a printer, or the density of device-addressable points.

* Sensor resolution: in pixels or megapixels, measures the total effective number of pixels available on a photosensor.

* Lens resolution: in line pairs per millimeter (lpm), this measures the fineness of detail that can be distinguished to a given contrast.

The other reason for confusion is that we are not used to the virtual unit. That unit is the pixel, and it has no size until it is output.

The central notion, therefore, is that you think pixels and you count in pixels. As you get used to working with digital images, you get a feel for whether the number of pixels you have in the image is sufficient.

Broadly speaking, given images of average proportions, images smaller than about 150 pixels long cannot show anything but the broadest structure of a picture. For Web use, your images generally need not be longer than 720 pixels-which is also adequate for smaller prints (i.e., half the size of an average postcard). For fair-quality prints and output to A4 size, you will work in the 2000 to 2500 pixel range. For good quality work, the numbers rise to image lengths of 3000 to 4500 pixels and more.