Which 35mm 1.4 has the "flattest" focus field at close distance?

Well, your best bet is the Macro Planar f2, because that will give you almost the same depth of field as a 35mm f1.4 for the same field of view.  Actually, Olympus made a manual 50mm f2 macro , http://www.mir.com.my/rb/photography/hardwares/classics/olympusom1n2/shared/zuiko/htmls/50mm2.htm you would have to adapt it to a Nikon mount, see http://www.leitax.com/OlympusOM-lens-for-Nikon-cameras.html but it likely would end up costing you about half of a new Macro Planar.

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