I've just done an experiment with DOF and 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 tripod 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 DOF.
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 DOF 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?

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