The shape of the field of focus

Who knows what the geometric section of the area in focus looks like for the majority of lenses out there?

Is it a linear section such that all areas of a wall (for instance) will be in focus?
Or is it radial such that everything within a specific range from the lens is in focus?

Just another meaningless technical question to keep the gurus on top of their game.

The natural plane of focus for a lens is curved, which can be clearly seen in many cheap cameras. As price increases steps are taken to make the focus point fall on the film or CCD at all points, but this will break down at wide apertures and close distances for many modern zooms.

Try photographing a sheet of newspaper with a cheap zoom at open aperture and see the edges go out of focus when the centre is sharp.

Small errors are covered by the "depth of focus" which allows the position of the CCD to be slightly in front or behind without losing too much sharpness (compare with depth of field).

A Macro lens will be designed with a flat field, making it more suitable for document copying and other critical uses.

For distant objects there is not so much of a problem design-wise.

I think it was Minolta who made a Variable Field lens which enabled alteration of where the plane of focus lay. Very useful in very obscure circumstances, and it never caught on.