You can try from 3 different edge detectors: Canny, Sobel and LaPlace.
Canny's method will produce a true edge image (black & white) while the other two create gray scale image. Canny's edge detector accepts 3 parameters: blur radius, high and low thresholds.
The blur process is the same as in Blur/Sharpen function.
High and low thresholds can be adjusted to get better result.
For more info on edge detectors, you can visit Bill Green's edge detection tutorial at http://www.pages.drexel.edu/~weg22/index.html.