Many years ago I tried a simlar process, instead of using a long exposure and changing the fstop I used multiple exposures and changes the fstop several times during the time I made the exposures. The end result was that the image had soft edges as has been suggested by rbarker. I also experimented with moving the point of focus as I changed the fstop. The idea of the experiment was an attempt to introduce a sense of movement into the image. Some of the images looked similar to those produced when a zoom lens is zoomed during the exposure.