The other day, I shot some portrait photos with Rebel XTi. My ISO was set to 100 and my flash was set +2/3 on a 420EX with very soft diffuser. The shutter was set to around 1/4 - 1/8 with an aperature of 5.6. When I viewed the pictures on the computer, they were rough, blotch sort of pixelated on the dark sections of the photos. I was figuring there may not have been enough light.
What did I do wrong?