MTF and over-sharpening

This is something that bugs me: Tuning sharpening as to make MTF50 happy. The slanted edge is a test based on the ISO 12233 spec. Because the imagers I work with use 5x5 kernels for bayer interpolation and (further down the line) denoise + sharpening, the slanted edge ends up being spread on 5~6 pixels.

If I divide one cycle per 6 pixels, 1 obtain 0.16 cy/pix.

However, because sharpening is here, I can use that knob to increase the edge frequencies by adding over-sharpening. There is a trade-off: Once pixel values start clipping to black or white, the frequencies will not increase and will actually start dropping.

Here is a graph I made while working with some imagers. I had 8 sharpening levels to play with.

The "optimum" trade-off seems to be at "2"!

Here are crops of some of the edges after re-scaling them for better viewing.