Focusing - macro ring vs focus ring

[Josh Mitchell Frey]

I'm doing a shoot tomorrow on a 2/3" ENG-style camera (a Sony DSR-450). The script involves a disorienting dance sequence in a post-apocalyptic wasteland - and I'm wanting to find a way to cover this scene with my 9-155mm zoom lens set reasonably wide (to exaggerate the disorientation), while still achieving shallow DOF.

 

The lens I'm working with is a 17x fujinon lens with a macro ring.

 

I've found, in a couple of informal tests, that by deliberately defocusing my focus ring, and then correcting the focus using the macro ring, I can create extremely shallow DOF at the wide end of the zoom range (at around 40mm, on subjects around 1 metre away).

 

I'm just wondering whether this technique has been used before, or whether there's some reason why no one would even consider using it.

 

I'll most likely find out the hard way tomorrow..

But in any case, I'm curious, if anyone here has any thoughts.

 

Thanks

[Rob Vogt]

40mm isn't wide on 2/3" so you should be getting a shallow dof at that focal length at a reasonable stop. If you want a really shallow dof at a really wide focul length (I guess at 9mm, even though that's not super wide for 2/3" either) you'd have to be wide open and still be pretty close to the lens. I've seen fisheye lenses that have the focus marks for anything past 3' being infinity. Are you sure your macro ring isn't the back focus adjustment?