try a cv 15/4.5. it's cheap enough that you can still afford a 21, 28 or 35. personally, if I had to live with only two lenses, I could get by very nicely with a 28/2.8 elmarit and the cv 15. the two together will cost less than either the 18 or 21. if money is not an object, you can better advice from other folks here than me.