Before I even joined Kodak, the standard method comprised of slits, not knife edges. And the slits were used to create images with Xrays and with visible light as a measure of turbulence in the emulsions.

The light exposures were both negative and positive, and we used several slit widths. In the example it is 10, 100 and 1000 microns but I have used 1 micron as well. The height of these at different exposures will give the relative contrasts as seen in the second image. This difference represents say 35mm vs 4x5 images and thus you "see" the image differently.

In 35mm, the 10 micron line may represent a telephone line, but in 4x5 that might be a 1000 micron line.

So, we never (AFAIK) used knife edges because they were not very revealing in many ways.

Pictures courtesy of Mike Kriss.