Hello diego,
I am also testing out DOF feature (for mayall) and a couple of things I found out are;
Not only the shutter speed and f-stop are important, but also other points are important such as setting FOV (zoom), Focal distance, and scene size and placements of objects also affect amount of DOF you get.
I am doing some test contains models in the foreground, mid and background spaced at more than 100 units (in cm), with;
fstop = 0.5,
focal distance = 20,
focal length = 35mm (FOV = 54.432)
shutter speed = 50000 th of a second
objects placements are ;
foreground obj = placed at 1 unit from a lense
mid obj = about 20 units
background = more than 100 units
Maybe using MXCamera's center of interest checked and adjust by using Maya's camera's center of interest manip later would help find the focal point. ( Or you can also try camera's center of interest Unchecked and use the focal distance)
Try adjust FOV as well - narrower you get more of DOF, I think.
hope this helps,
-tak