hey bruce i also have a 50d and have noticed some focus problems to. i have a sigma 17-70 canon 70-300 III and canon 100-400 L the 100-400 works best for me but still get some bluring but my sigma seems to allmost allways need correction.i want to get a 7d or mabe 5dII so ive been a littlle concerned about this but gald to hear good reviews on 7d thats the one i like most.....rod