Wednesday, 15 April 2015

Trust, but verify

A while back I wrote:
  "I could not find any synthetic images rendered with specific, exactly known point spread functions. This meant that the only way that I could tell if MTF Mapper was working correctly was to compare its output to other slanted edge implementations." (here)

This was the main motivation for developing mtf_generate_rectangle.

It turns out that Imatest recently updated their SFR measurement algorithm to include the well-known finite-difference-correction (here). I first encountered this correction in one of Burns' papers. According to the Imatest news article, this correction is now included in the ISO 12233:2014 standard.

I have yet to test the new version of Imatest against the synthetic images produced by mtf_generate_rectangle, but I do recall that some quick-and-dirty testing a few years back hinted at Imatest underestimating MTF50 slightly, which would be consistent with an algorithm that does not apply the finite difference correction. As pointed out in the Imatest news article, this difference is really only noticeable when dealing with higher MTF50 values, so this does not imply that all older Imatest results are now suddenly obsolete.

It does raise an important point about traceability and independent verification, though.