I would be careful with the use of the word 'resolution' as this really relates to the pixels x pixels size of the shot. As you mentioned, at 1.8 your not leaving much depth of field to be in focus and so are loosing sharpness. With a 50mm at 1.8 and say about 1m away from your subject, your depth of field would only be around 3cm. That doesn't leave much depth to keep a face in focus, not unless your subject is looking straight at you and even then you could loose the nose and ears (assuming your focusing on the eyes). I had the same problem recently when shooting with my 50 and opened up the aperture to blur the background (and forgetting about the effect on my subject).
I would recommend playing around with this site
http://www.dofmaster.com/dofjs.html and familiarise yourself with your lenses capabilities. Select your camera, focal length, aperture and distance then after clicking calculate, check out the depth of field total on the right.
Hope that helps
G