Wavefronts and the cartesian sign convention

Wavefronts that converge to an image are taken as having a positive curvature and so if the object if to the left of the lens the distance from the lens to the image is positive.

A lens adds a constant curvature (1/f) to the wave.

The Cartesian sign convention takes distances to the left of the lens (or mirror) as negative and those to the right as positive