If you can find a 2x converter that goes in front of the lens you will maintain the f/1.8 as far as focussing goes - ie a 50 mm f/1.8 lens will turn into a 100 mm f/1.8 lens, albeit of less-than-perfect optical quality. There will be some extra transmission loss, as with a behind-the-lens converter, but that doesn't affect depth of field. The difficulty would probably be in finding such a converter that didn't vignette with a 50 mm lens. A lot of tele-converters were made for Super 8 mm movie cameras with non-interchangeable zooms, and they are available second-hand. New ones of reasonable quality are fairly expensive. The Century one, with a 58 mm mounting thread, is 500 bucks new.