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This image of my MTF test chart was supplied by Sergio Lovisolo.Some of the problem is due to the Bayer sensor: the effective horizontal and vertical Nyquist frequency for red and blue pixels is half that of green.Many digital cameras have optical "anti-aliasing" or "low-pass" filters in front of their sensors.These filters spread the light, increasing the value of n in the sensor's response.The corresponding spatial sensitivity functions are shown below. But when an average of possible phases is taken, the MTF curve is a good match to |Sinc( f /dscan)|The first sensor null (the frequency where a complete cycle of the signal covers one sample, hence must be zero regardless of phase) is twice the Nyquist frequency.The spatial sensitivity function for sinc( f /dscan) is a rectangle of length 1/dscan (the length of a sensor element with a fill factor of 1). Fagard-Jenkin, and "A Novel Approach to the Derivation of Expressions for Geometrical MTF in Sampled Systems", by R. The sensor's average response (the average of all sampling phases) at the Nyquist frequency can be quite large.Kodak publishes MTF curves for a low cost anti-aliasing filter. MTF response is outstanding, but remains high (0.31) at the Nyquist frequency.
The best flatbed scanners in Don Williams' paper had MTF = 0.3 at the Nyquist frequency— close to the response for n = 3.You can learn more about film scanners— specifications and a product list— on the Scanners page.Now, let's look at actual scanner resolution, starting with the dpi specification.In all fairness, this aliasing is rarely objectionable in real images, though it could show up on fabrics.Kodak has some software solutions The sensor manufacturer, Fill Factory, has an interesting list of publications including an excellent paper on this sensor.x) otherwise).
The figure below illustrates how response above the Nyquist frequency leads to aliasing.