If the faces are blurry then my guess is that they are not all within your depth of field.
let's assume that you have your 50mm F1.8 mounted and have F4 selected.
To get a group in I am guessing 10 feet away- then your depth of field is about 2 feet- 1 foot in front and 1 foot behind your point of focus.
At F8 it is nearly 4 feet.
Have a look at dofmaster (link below) and use the alpha 550 (580 not listed but the result will be the same).
Have a play varying the parameters in dofmaster to get a feel for what changing your settings will do.