I've just done an experiment withand thought I would share the results to double check I have this right.
The subject was a friendly raptor and I put the camera on a and cycled through the settings.
The first group is at 55mm and I went from f/5.6 through to f/36 to see the change in .
For the second line I moved the camera slightly back and repeated and I noticed that even this slight change made a significant difference in the result.
The second group is at 18mm and f3.5 to f/22. The change in is there but not so obvious as it was at 55mm.
One thing I was not expecting was how much I had to get in the raptors face to make this work. I can now also see that the focal length plays an important role in this process. Are there any other setting I should be looking at with this experiment?