Accurate edge detection?

I originally posted the following on the sci.optics newsgroup because
it's more of an optical query.  Nevertheless, the folks here may be
more familiar with the available references and research that's been
done. Please forgive my statement of some things that will no doubt be
obvious to many here.  (BTW: I also have a description of the
experiments we've performed for anyone interested.)

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In the image processing community, there are discussions of edge
detection techniques.  For a simple case, imagine an opaque knife
edge
with a uniform back light behind it. Further imagine there is a lens
that images the knife edge onto a pixel detector of a camera.

In general, the transition from dark to light at the detector is some
smoothly varying function, not a sharp jump.  Diffraction, of course,
limits the ultimate sharpness of the edge image -- diffraction at the
edge itself, and diffraction at the aperture of the lens.
Aberrations
of the lens will also contribute to this edge smoothing.

The digital image as presented *by* the camera may take only a pixel
or two to transition from dark to light, or it might take many more.
Regardless, digitization and pixel size and other factors such as MTF
of the electronics themselves serve to mask the true edge function

A very common starting point in the discussions and papers about edge
detection techniques is the assumption that the point at which the
slope of the edge is maximum represents the "true" edge position.
From then on, the various edge detection algorithms usually present
different methods of more accurately calculating this maximum slope,
especially in the face of optical and electronic noise, etc.

Nevertheless, it seems to me that the assumption that the maximum
slope represents the true edge is at least unmotivated (no matter how
"common sense" it feels) if not wrong.  I have in mind the pictures
of
edge diffraction as produced by using Cornu's spiral.  The location
of
the true edge, relative to the average intensity of the light area
(smoothing out the diffraction oscillations), looks to be at a
position of increasing slope as you go from dark to light, but NOT
maximum.  Furthermore, the edge is less than the 50% point of peak
light intensity.  (Another assumption sometimes made in image
processing is that the 50% point of the dark to light transistion
represents the edge.)

Does anyone have references they can point to (or their own pesuasive
arguments) that describe where the true edge location should be
relative to the edge image function?  We have done a couple of bench
tests to suggest under the experimental conditions that the best edge
location is about 41% to 46% of the range of the dark to light
transition.  We haven't yet completed our analysis regarding how this
compares to the peak slope.

This problem has to have been tackled successfully before, but so far
I've not found any good sources that address the optical issue, only
software techniques.

Thanks!

Spencer