What lens or technique would work best in this situation?

[Leonard Evens]

Maybe I am doing the calculation incorrectly, but I get between 3 and 4 mm DOF for a 210 mm lens at f/5.6 on a 4x5 camera focused at 2 feet (610 mm). It wouldn't be much different for an 8 x 10 camera.

I also tried it with my 4 x 5 camera using a 210 mm lens set to f/5.6. I focused on something at 2 to 3 feet. The background, about 100 feet away was pretty out of focus. I could make out general shapes but no detail.

So I don't know just what you want. It might be pretty hard to arrange things so that the background was just a complete blur with nothing whatsoever showing. For example, the hyperfocal distance of a 210 mm lens at f/5.6 using a CoC of 10 mm is about 788 mm. With that criterion for sharpness everything from 394 mm to infinity would be "in focus". A 10 mm CoC would be a pretty coarse criterion for sharpness, but it would still allow you to detect general shapes.

I think the only way to arrange to have nothing at all showing in the background would require the use of a photeditor to blur the background.

[Jac@stafford.net]

Maybe I am doing the calculation incorrectly [...]

I seriously doubt your calculations are in err, Doctor Evens. (Presuming you are the Northwestern U mathematician I recall.)

To answer the OP question - to be perfectly ethical, I would disassemble the stone wall and rebuild it farther back from the trees.
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