Originally posted by maltfalc the effect is caused by deeply recessed front elements or other design aspects that begin to block the light path as you move away from the center of the image. mounting a step-down ring or too long or narrow hood on your fa 50 will make it swirl.
Not completely true, as using just the lens block from any swirly Helios, MOG, LOMO or Voigtlander will give swirls with nothing obstructing the light path in front of the lens. On the other side I do believe that with certain lenses your theory might work.
---------- Post added 12-13-16 at 10:42 AM ----------
My swirly favorites (in 30, 50, 58 and 92mm focal lengths):
Pentacon 3.5/30mm (MOG Lydith 3,5/30)
Aster tripolium by
Marko Jankovic, on Flickr
Carl Zeiss Biotar 2/58
Phragmites australis bokeh by
Marko Jankovic, on Flickr
Leti-60 projector lens 2/92
Pharmacy in Valjevo by
Marko Jankovic, on Flickr
Voigtlander Ultron 2/50
backlit Tropaeolum by
Marko Jankovic, on Flickr
and maybe my favorite, Steinheil München Cassarit 2.8/50
KO-120M 120mm/1.8 will give Cassarit like bubbly swirl at a great FL of 120mm, but I can not dig out a picture made with it at work.
All these lenses are affordable (maybe Biotar with a bit of patience), I paid no more then 35 EUR for any of these and for some, half that price. For some of these, use on Pentax calls for some adapting but nothing too difficult or expensive.