I mean use the Free Transform tool to expand the width at the top and compress it at the bottom in order to correct for perspective. When expanding you loose resolution so 2 shots with 50% overlap effectively increases the DPI to compensate for what you loose in PP.
I understand what you mean now Paul. I usually use transform / distort and work inward when I need to correct perspective using software.
Hamish needs tilt rather than shift to after the focal plane and therefore gain DoF for product shots. One of the options would be a tilt/shift lens. These are expensive and offer limited movement on a DSLR. There is much more scope with a technical camera but to combine these with a digital capture back would probably cost at least £10K (but you get full movements on both the lens axis and the film/sensor axis). A cheaper option might be to use a S/H 5x4 technical camera (lets say £200 - £300) plus a lens (another £200) plus an adapter to fit a DSLR in place of the screen/film holder/back (the last link was to a firm in the US that makes such adapters - I'm sure there are others). You can buy cameras that are designed for this but they cost about £3K plus the lens.
I thought there must be someone who has done this as a DIY project and ...