DaSigmaGuy explains at dpreview.com in the Sigma forum how to convert Canon FL 55mm to Sigma SA mount. Click for youtube video
I would like to convert an FL 58mm to compare with 55mm. Both are common on ebay. 58mm often costs more, but either will provide the most sharpness and creamy bokeh for the money.
58mm has an f-stop preset stop ring, while 55mm will be locked in Manual mode during conversion. Twisting the aperture pre-set stop ring may make it easier to stop down while looking through the viewfinder, since you don't have to mentally count clicks, but 55mm click counting is a single step, interacting with one ring instead of two.
