The resolution parameter is always declared by the manufacturer in the scanner’s technical specification. However, the real optical resolution is determined not only by the number of the elements of the light-sensitive sensor, but also by the quality of the optical system of the scanner. Good resolution comes from the scanner’s ability to catch the minutest details in the original. But a scanner with a higher optical resolution doesn’t necessarily produce scans of a higher quality.
We can check this parameter out in practice by digitizing special test targets. Such targets are sets of patterns of white and black lines. The spatial frequency characteristic of a pattern is given in the number of pairs of such lines per length unit – line pairs per inch (lppi) in our case. The next picture shows the test target I use:
There are five patterns at the top of the target which differ in their line density: 30, 75, 95, 140 and 180lppi. I will scan them at the maximum optical resolution of the scanner.
The image contrast tends to degenerate at higher spatial frequencies. This tendency is characterized with modulation (as explained in my previous reviews). 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 for two other scanners for the sake of comparison.
The modulation transfer function (MTF) is calculated by a similar formula. The MTF is the frequency characteristic of the scanner’s optical system.
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: