You need a portrait lens to take portrait photos. Portrait lenses usually have a small aperture number, like f/1.8. This ensures a natural background blur which makes the person stand out from the background. The focal length you need depends on how much compression and background blur you want. Choose an 85mm lens for little compression. With a 35mm, you can capture more of the surroundings. Keep the mount in mind when you buy a portrait lens. You can choose between portrait lenses from Nikon, Canon, Sony, Sigma, and other brands.