Where you place the camera relative to the subject, the lighting and the lens’ focal length gives you the ability to “interpret”, say something about him or her, the subject.
- To denote the power of the subject you will take the photo pointing
your camera slightly upwards.
- To denote submission you will take the photo being above and
pointing slightly downwards.
- To denote a sinister subject you can take the photo pointing >45°
upwards and having a hard light fall on the subject from a high
position from your “back” (his 0°) and downward >45°.
If you are taking a normal portrait and not trying to editorialize about the subject, the most common setup is to have the lens around (you can go a few degrees lower or higher specially with telephoto lenses) the height of the noise (middle of the face) of the subject and use a telephoto lens (from 85mm to 135mm Full Frame equivalent) to avoid the distortion caused in the facial features of the subject by being too close to the the subject when you use focal lengths below the 50mm.
The longer the lenses the further away you will be from your subject and the more leeway you will have to stay in a confortable position while shooting and you will still be just a few degrees below or above the nose of the subject.