How a Dove prism transforms the orbital angular momentum of a light beam.

It is generally assumed that a light beam with orbital angular momentum (OAM) per photon of lh, is transformed, when traversing a Dove prism, into a light beam with OAM per photon of -lh. In this paper, we show theoretically and experimentally that this OAM transformation rule does not apply for highly focused light beams. This result should be taken into account when designing classical and quantum algorithms that make use of Dove prims to manipulate the OAM of light.