The resolution is the accepted criterion for dividing scanners into different categories. This parameter is indeed an important characteristic of any scanner. The real optical resolution depends not only on the number of elements in the CCD array but on the entire optical system.
It is possible to check how well the scanner digitizes an original with numerous tiny details. For such tests special patterns with alternating black and white lines are employed. The spatial frequency parameter of a pattern is expressed in the number of line pairs per inch (lppi). A pattern looks like that:
Applied Image QA-69-P-RM Test Target
There are five patterns here with a different spatial frequency (30, 70, 95, 140 and 180lppi) and I will scan them at the maximum optical resolution of the scanner (4800dpi).
The image contrast tends to degenerate at higher spatial frequencies – this tendency is characterized with modulation. By measuring the modulation for the five patterns of the target it is possible to see the dependence between the image contrast and the spatial frequency of the patterns. I publish the results of two other scanners for the sake of comparison.
Next, I will calculate the modulation transfer function (MTF), which is the frequency characteristic of the scanner’s optical system (see my earlier reviews for details about the MTF calculation method).
The graph above shows you the MTF for the test patterns. I took the leftmost, 30lppi pattern of an Applied Image QA-69-P-RM target as the reference area. The MTFs are calculated for the green channel of the target image. The following MTF diagram is based on the obtained data:
The diagrams suggest that the Perfection 4490 is as good as its device category and delivers the declared optical resolution.