Likeness in portraiture, a challenge and a source of trouble.
A good portrait must fulfill the two basic requirements: perfect similarity and good artistic characteristics, and it seems that sometimes the two are at odds. I know from experience that I was more prudent in my approach with commissioned portraits than non-commissioned artwork and the more I progressed during the years the more I was able to grasp at these things. But still sometimes it is difficult.
The actual likeness depends on the large forms and volumes in the face, not on the accumulation of possible details. Take the test with your school photograph. The faces are not bigger than one-third of an inch, and still you know who is who. No details, no wrinkles, no skin colour, no colour of the eyes or hair!
How to get a grip on the likeness?
1. A good light-dark contrast in the face helps to obtain likeness.
2. Use a mirror and see the model and the canvas at the same time.
3. Make a photograph of your portrait and see it on the screen in a mirror image.
4. If you wear glasses, take them off to see the big volumes and values.
5. Check your portrait in dark conditions you will see different things in the half-light.
6. Check your painting with fresh eyes: Take a break off and on.
Again for clarity: Adding more details to the portrait does not mean adding more likeness, less details make it easier.