Most people are not going to spend the money for a Canon EF 400mm f/2.8 L IS II USM Lens for only portrait photography use. But, this is an incredible portrait lens. If you have one of these lenses, don't be afraid to assign portrait duty to it.
For this image, I had the girl and dog positioned against a distant fall-colored hillside with a modestly cloud-diffused late day sun providing nice lighting. Green grass anchors the frame.