Depth of field

I love this Picture.  The really narrow depth of field makes the flower seep to pop off of the photo almost like 3D  I took this with my Canon XTI and 28-135 lens.

F-stop.  f/8
Speed.  1/250 sec.
ISO.     100
Focal Length.  135mm
Subject Distance 400mm

The Minimum focal distance for this lens is published at 500mm.  I am able to get focus confirmation at 400mm with my copy.  I wonder if other 28-135 Lenses are the same. I

This photo was shot in Raw.  Cropped square and converted to Jpeg.  no other editing was done.