Can conjugate images be used in place of dark field filters?

[Ruddiger28]

I was working on some dark field filters and I was wondering if you could substitute the black spot in the centre of a dark field filter for a completely black filter with a light passing hole in the middle. Sort of an inverted dark field filter.
I seem to remember reading something like this involving conjugate images when I was doing Fourier theory a while ago.
Would the filters have the same effect? Or are conjugate images Fourier transforms of each other?

 

[Andy Resnick]

In a darkfield imaging setup, you cannot substitute a small hole for an annulus; the annulus passes high spatial frequencies while the hole passes the DC component. Offsetting the hole to the radius of the annulus can be acceptable, however (google: synthetic aperture radar).

 

[Ruddiger28]

Thank you for replying, I was afraid that was the case. So on the conjugate images point, are the two filters in this case Fourier transforms of each other? Why are they called conjugate images? Is the link between the two simply that they appear as inverted forms of each other or is there something deeper?

 

[Andy Resnick]

Are you referring to phase contrast imaging? Then your question makes more sense:

http://www.microscopyu.com/articles/phasecontrast/phasemicroscopy.html

also, there's a pattern recognition technique using 'matched filters':

(can't quickly find a convenient link...)