K20d D range optimiser

I notice on my new 20d that it has an d range optimiser, when pressed I lose the option if 100 iso, have taken images with it on and off and cant tell the difference, any ideas>

I guess you mean Dynamic Range (page 79 in manual)? It expands the sensor's dynamic range (or it should). The only effect I noticed I had much more noise, particularly in dark regions, even with low ISO set, so I happily switched it off and never used again.

Cheers for that Michal