You've already got the 24mm, but you might want to keep an eye out for that Canon 17mm lens. It gives you a perspective that is significantly different from either a 24mm or 20mm. I got mine, a newer FD (bayonet) copy, at an estate sale for $150. After that, I got an F-1 body for $50, just to be able to use it. At first, I was going to mount it onto a Bessa L, but I had no external viewfinder that was even close to suitable for that wide a lens, plus it felt so huge on the Bessa.