I shot about 500 pictures today and found that F25 can give you a bit too much depth of field... I guess there is no safe F-Stop... It changes from shot to shot... Check to see if your camera has a F-stop Preview... I forget what the manual calls it right now.

On my camera there is a switch I can throw that will open or close the iris down to the proper setting so you can see if the depth of field is okay... Still a pain to use... Makes the view finder dimmer... It is really designed for far away zoom and not close up zoom, but it does work.