I have a Sigma 100-300 f4 EX. When I first got it, it focused fine. Now\nhowever it has gone soft on me. At f4 it will focus fine at 135 and 200mm\nproducing a very sharp image but not so at 100, 250 and 300 with 100 being\nvery blurry. Stopping down improves the situation a bit. I am using it on a\n20D and a D60. It behaves virtually the same on both. I contacted Sigma and\nwas advised to have my cameras' 'focus flange distance' (FFD) checked.\n\nAm I right in thinking that if this FFD were wrong, then it would not focus\nat any focal length? Also that it is highly unlikely both cameras would have\nFFD errors, suddenly.