A good head and shoulders portrait is the combination of a number of factors.
To ‘isolate’ the subject matter the background needs to be blurred, whilst maintaining a pin sharp subject. This is obtained by positioning the subject some way in front of the background, and taking the photo close to the subject. By using a long focal length of about 100mm in conjunction with a wide aperture you create a shallow depth of field with a very soft background.
f4.5 @ 1/500sec ISO 800, focal length 105mm, distance to subject 1m. Lens 24-105mm
I see! 