Composing a Portrtait

This article merely aims to highlight the key "rules" of composition, and focuses primarily on a head and shoulders portrait of a single sitter.
Principle point of interest
Any image requires a principal point of interest, and in a portrait this is usually the subject's eyes.
The principal point of interest should not be dead centre in the frame: this is too static and symmetrical. Imagine a line running through the subject's eyes, and another at 90° running through their nose and chin to form a cross. The best position for this cross is slightly above the horizontal middle of the frame, and slightly to the left or right of the vertical middle.
Whether the cross is slightly to the left or right depends on which direction the subject is looking. If they are looking to the left, there should be more background to the left, so the cross is placed the the right of centre. Conversely, if they are looking to the right, the cross should be placed to the left. This helps prevent the viewer's eye from being drawn out of the picture when it follows the subjects sight line.
When the subject is facing directly towards the camera, consider the direction of the shoulders. Here the aim should be to have a little more space in front of the subject rather than behind them. So, if the right shoulder is nearest the camera, frame the shot so their eyes are also right of centre.
Returning to our imagined cross on a face, if this is upright, the portrait will appear very formal. This is often a good option for more mature subjects. Tilting the cross to either side creates a diagonal axis to the features, which suggests movement an vitality, and is often best for younger subjects.
Triangles
Composition is always improved by the creation of triangles. When composing a portrait, try to visualize the outline of the subject fitting into a triangle.
Tall triangles in an upright format suggest height and dignity. In a landscape format they hint at confinement or struggle. Low triangles in a landscape format imply stability. In an upright format they give an impression of weakness. Triangles tilted forward suggest movement, and titled backwards they imply liveliness. Tilt too far in any direction, and stability will be lost unless a second stabilizing element is introduced. If someone looks like they are about to fall over, it should tell you that the composition of the shot is wrong (e.g. A head tilted greatly to one side may need a hand to support it. Introducing an arm bent at the elbow creates another triangle and so restores stability).
Lines can be broken so long as the eye can easily skip them. Lines can also be imaginary, or implied. For example, a viewers eye will always tend to follow the subject's line of sight.
Tones
Our eyes are always more attracted to light tones than dark tones. Directional tendencies can be created by tones, so for example, if there are two or more adjacent light areas, the eye will travel from one to the other.
In a portrait, the face should be the principal attraction, and therefore lighter. Any other light area (clothing, hands) need to be subdued in tone, Avoid strong patterns (e.g. clothing), as these can compete with the face for attention.
A portrait needs to be tonally balanced. This means not having all the dark tones on one side, and all the light tones on the other (Unfortunately, this is what we see in a typical wedding photo of the Bride and Groom!).
Backgrounds
Working as a portrait artist, I rarely reproduce the background in a reference photo, for good reasons, and choosing the background for a portrait is probably worthy of a dedicated article (see Backgrounds).
All I am going to say here is that the simplest consideration is either a light or dark background. Dark backgrounds are commonly see in classic portraits. Light backgrounds are currently trendy in modern photography. For my purposes, a light background works best, because it is makes the outline of the subject easier to see.
Colour
Colour harmony and behaviors are an important element of composition. Colours convey mood and depth. Cool colours appear to recede (blue-greens, blues, purples), while warm colours appear to advance (yellows. oranges, reds). Cool or neutral colours work best for backgrounds, while warm colours in clothing help give a portrait depth.
Colour choices depend on the subject. Blondes and brunets have differing requirements; blue works well with the former, while blue-greens/greens better suits the latter. Somber colours suit older subjects, while vibrant colours go well with young subjects.
The key aim should be to choose colours that compliment, and are sympathetic to, the subject's colouring.
Footnote: There are circumstances when these "rules" may be broken, but the intention of this article is to convey simple guidelines applicable to most situations.
Portrait photography from a Portrait Artist's perspective. Portraits by John Burton