Replying to help with photography

ok, so i have been testing out this DOF stuff i recently heard about on this site. with my canon A70, i was testing it out. I shot a pencil at close range, f at 2.8 to get the pencil clear and the background blured. I then moved the focal to 8.0 and took the same shot to try and increase the DOF and make the background more clear. All the time letting the camera select the shutter speed. because the change in focal lengths, there was not enough light to properly expose the shot. so i turned the ISO setting from auto to 400. this made the picture the correct light i wanted, but it also made it really really gainy. lots of noise in it. is there a way to take this shot by adjusting other settings or am i stuck with the grain in this kind of camera?
MAT