Resolution is usually listed by the manufacturer in a scanner specification, but the real optical resolution depends not only on the number of elements in the light-sensitive device but on the optical system, too. Good resolution means the scanner’s ability to reproduce all the minutest details of the original and sometimes a scanner with a high optical resolution may produce a worse-quality scan than a model with a lower optical resolution.
This can be tested in practice by means of special patterns with alternating black and white lines. 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 8400F scanner.
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. 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.
The graph above shows you the MTF for the test patterns and corresponding histograms. 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: