For measurement of sharpness, the main driver of variation is noise. A powerful noise reduction technique called modified apodization is available for slanted-edge measurements (SFR, SFRplus, eSFR ISO and SFRreg). This technique makes virtually no difference in low-noise images, but it can significantly improve measurement accuracy for noisy images, especially at high spatial frequencies (f > Nyquist/2). It is applied when the MTF noise reduction (modified apodization) checkbox is checked in the SFR input dialog box or the SFRplus or eSFR ISO More settings window.
Note that we recommend keeping it enabled even though it is NOT a part of the ISO 12233 standard. If the ISO standard checkbox is checked (at the bottom-left of the dialog boxes), noise reduction is not applied.
The strange word apodization* comes from “Comparison of Fourier transform methods for calculating MTF” by Joseph D. LaVigne, Stephen D. Burks, and Brian Nehring of Santa Barbara Infrared. The fundamental assumption is that all-important detail (at least for high spatial frequencies) is close to the edge. The original technique involves setting the Line Spread Function (LSF) to zero beyond a specified distance from the edge. The modified technique strongly smooths (lowpass filters) the LSF instead. This has much less effect on low-frequency response than the original technique and allows tighter boundaries to be set for better noise reduction.
*Pedicure would be a better name for the new technique, but it might confuse the uninitiated.
Modified apodization: original noisy averaged Line Spread Function (bottom; green),
smoothed (middle; blue), LSF used for MTF (top; red)
As of version 5.0.17, additional filtering to increase smoothness of MTF curve in noisy images has been applied.