lens - What swirly bokeh technique is this and how can I achieve it?

What is the technique used to capture this image? How can the result be achieved? I know that it can be achieved in a digital darkroom but how can I replicate the effect straight out of the camera?

Answer

This photo is taken with a petzval lens which corrects all aberrations decently except for, well, petzval aka field curvature. Because the edges are in focus at a further distance, the blur is smaller there. Because the lens is fairly highly vignetted, the lens also effectively has a larger f number towards the edges, again reducing the blur. The result is the swirly bokeh you see in this photo.