Its always difficult to answer what is the best focal length for a portrait. Here is small preview how different focal lengths can change look of a face.
The lens or Focal length of the lens you choose has a profound impact on the image. It effects the perspective, background, and features of the subject. Everyone should understand what the different focal lengths do to the human face and how they change the background. Why do you choose a 135 mm lens to shoot a head shot of a beautiful woman but use a 24mm to shoot a clown?
Keeping the head the same size in the frame and changing the focal length of the lens, here are some shots of images that demonstrate how focal length effects the features of the human face and how it changes perspective which effects the background. In this test we see that longer focal lengths flatter the face and shorter focal lengths make it look distorted and comical. I prefer to shoot a head shot with a 100mm to 135mm focal length lens on a 35 mm camera. This keeps the face from distorting and gives a nice look to the subject.
When shooting on a set there is no set rule when it comes to lens focal length. Longer lenses need less set and shorter lenses need the subject to be pushed close to the background or a larger background. If you keep the subject the same size in the frame when changing the focal length it will change the perspective of the image. Things push farther away from the camera with shorter lenses and come closer with long lenses. The person’s face also will change just as we saw in the first test. We will need to balance the look of the person’s face and body with the perspective we desire in the background. I love to play with different perspectives on set, using different lens.