How to squeeze the utmost quality out of your film/scanner combination.
By Tom Ang
Another source of confusion is the variety of approaches that manufacturers take in designing scanner-driver software. Some require that you set a scaling or enlargement factor, while others ask for the output size-and some allow you to set both. Worse, as you change one figure, others mysteriously change or else you are forbidden to enter certain numbers-usually for reasons that are never explained.
Part of the reason is that scanner drivers are helpfully written to prevent users from creating impossibly large files, and to discourage odd resolution settings that will involve a great deal of interpolation. You learn as you go.
It helps to understand the relation between the scaling factor or output size that you set. Suppose you scan a 1-in.-sq section of a transparency. If the scaling factor is, say, 4x or 400%, the output size will be 4-in. sq. A factor of 6.73 produces a square of 6.73 in. Alternatively, if you set an output size of 4-in. sq, the scaling factor will be 4x or 400%. Personally, I find it unhelpful to set scaling factors and find it easiest to set output size.Note another complication, however. These are the chain-link symbols, which indicate that a pair of settings are locked to change in step (i.e., to keep their proportions). If you have already set a crop on the pre-scanned image and have locked the crop, the scanner software will only accept settings that maintain the crop proportions.
Now, if you set an output resolution of, say, 200 ppi, then the input resolution must change as you vary the scaling factor or output size. The input resolution increases to provide you with enough pixels to meet the output resolution that you seek. And when that resolution hits a limit it cannot increase to match, you will find that you cannot enter the scaling factor-it is forbidden. Say, for example, you want a 4-in.-sq image output to 200 ppi, then you need 800 pixels. The original 1-in. image must be scanned at 800 ppi.