Lens Focal Length
Medium to short range telephoto lenses are some of the best lenses to use for low key portrait photography. A lens with a focal length ranging from 85mm to 135mm will reproduce an image that accurately represents the subject.
Aperture Selection for Shutter Speed
Fast camera lenses, those that have large apertures to allow more light in, are good for low key portrait photography as they allow a photographer to use a faster shutter speed. While you should use a trip-pod for portrait photography, if the shutter speed is too slow, the picture may appear out of focus from camera shake.
Aperture Selection for Bokeh
Select a lens that has a large aperture for the most bokeh. Bokeh is the soft, out-of-focus background that is the result of a very shallow depth of field resulting from a larger aperture opening. This technique draws the viewer's eye to the subject who will be in focus in the picture.
Image Stabilization
Image stabilization is a good feature when you are using a long telephoto lens and may not have a tripod or monopod to support it. Image stabilization may be available on lenses in this focal length, and while it is not necessary it can be an advantage to the photographer who likes to move around his subject when shooting as opposed to being in one shooting position.