What lens or technique would work best in this situation?

[C. D. Keth]

I can envision few situations in which you would want to
"change the distance and maintain the same framing".

- Leigh

Really?

[Daniel Stone]

Using a longer FL lens will allow for more 'compression', and because of the longer FL lens having less DOF due to your close working distance, the DOF will be further cut down.

I've attached an image showing how FL relates to background 'blur', please note that the same f-stop has been used on all 3 shots, irregardless of FL of the lens used.

Note: I sourced this image from the www, it isn't mine. It simply demonstrates what I'm referring to.

[Brian K]

People here do realize that switching to a much longer focal length will greatly affect the perspective between the near and far subjects don't they? If the right focal length for the shot is x in order to get the composition right, then x it is. Putting diffusion over the lens or some sort of weird partial diopter correction is also going to be very imprecise and look very fake. If movements can't do it then the simplest solution is to do it as two shots. One capture of the wall in focus, and then another exposure with the focus shifted to make the tress soft. Given that the wall is a hard edge it is a pretty simple PS photo compose, or an enlarger masking situation.

[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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