Problem is Rob it's not "exactly the same" that's the point the overall DOF might be very near to the same in some cases but the front and rear DOF is not depending on the focal length used. The article does not mention this at all. (DOF 2) The rule does not apply at all focal lengths as we can see here mainly based on hyper focal distance:
http://www.dofmaster.com/dof_imagesize.html
As you can see it's not a cast iron rule.
Shorter focal lengths will have a larger DOF behind the focus point than in front of it and longer focal lengths will have a more even distribution.
But more to the point I'm running into folks who say focal length has nothing to do with DOF and I'm not sure the articles on the site help in this regard.
“Problem is Rob it's not "exactly the same" that's the point the overall DOF might be very near to the same in some cases but the front and rear DOF is not depending on the focal length used. The article does not mention this at all. (DOF 2) The rule does not apply at all focal lengths as we can see here mainly based on hyper focal distance:”
Look, I’m not good enough with the non-photographic bits of PS to draw this for you, but let’s see if I can do it in words.
Imagine a diagram that shows a shot taken with the widest lens (cone) you have. Then, change the acceptance angles inwards to show the different space covered by longer optics (narrower cones). If you do not move the camera, all that using longer lenses does is narrow the view and area covered and increase the image size.
But the relationship between background and subject has not been changed, you are just showing less of that background. Keep the image size the same as in the original shot and nothing changes other than you now have a greater camera/subject distance.
In other words, depth of filed is the result of image magnification, aperture being kept constant.
Sorry if this doesn’t explain it, but it’s as good as I can write it!
Rob C