The answer to this question depends largely on your personal preferences, the camera you own, and your budget.
Zoom lenses, especially high quality ones can be remarkably good at shooting portraits or prime lenses are not an obvious choice. You need to look at the details, such as aperture and image quality.
Prime lenses do tend to be much faster, that is have a wider maximum aperture than comparable zoom lenses. This is great for portrait photography as it allows you to produce beautiful out-of-focus backgrounds with a shallow depth-of-field. Wide aperture is also extremely important when you work in low-light conditions.
Image sharpness is another area where prime lenses outperform zoom lenses in the same price range. Primes are usually more simple, contain fewer elements and thus there is less potential for distortions or flares.
The price difference between prime lenses in the same range can be often very steep. As a rule of thumb, expensive models will give your higher quality images, and they will be built to last many years. However, due to metal bodies and thicker glass, they tend to be heavier than the cheaper plastic lenses.
There are three main types here: wide-angle, normal and telephoto lenses.
Wide-angle lenses are great if you want to take a photo of the subject with their surroundings (environmental portraits). They are not so good for close-ups as they tend to produce edge distortions.
Normal lenses, with a focal length of around 50mm, give a similar perspective to those experienced by us humans.
Short telephoto lenses of around 85mm are perhaps most used prime lenses portrait photography. They provide an excellent flexibility of taking close-up photos as well as photographing the whole person.
If you have a APS-C camera body, a 50mm prime lens will also fall into a short telephoto range.
Telephoto lenses allow even more ability to isolate the model from the background. However, they tend to be heavier and much more expensive.
In the list below you will find some of the most popular portrait lenses available today:
This super-fast wide-angle prime lens is one of the most respected Nikon has ever produced. The f/1.4 maximum aperture permits handheld shooting in low light and delivers a striking separation between subject and an out-of-focus background.
It's been optimized for edge to edge sharpness and includes the latest advances in optics, such as aspherical lens elements to reduce aberrations or nano coating to eliminate ghosting and flare.
Nikon f/1.8D is a high-speed prime lens with a focal length of 50 mm. It's small and versatile, with superb low-light capability which makes it great for portrait photography.
The minimum focus distance is 1.5 ft.( 45 cm
Nikon f/1.4D is an ultra-fast prime lens, which allows shooting in almost all light conditions. The focal range of 50mm makes is especially suitable for full-length portrait photography, for example at weddings. Nevertheless, it is also popular among travel photographers as well thanks to its crisp images and color integrity.
This prime lens from Nikon has a 105mm focal range and has been designed with close-up and potrait photographers in mind. It features image stabilization (VR II) and can focus from as near as 1ft away. (ca. 30cm).
The lens body employs extra-low dispersion (ED) elements as well as nano crystal coating to help reduce chromatic aberrations and flare. It weighs 27.9 oz. (790g)
The Nikon 70-200mm was designed with professionals in mind. It is a great lens for sports, wildlife and photojournalism. It contains seven ED (Extra-low Dispersion) glass elements and nano crystal coating to provide exceptional image sharpness and clarity with dramatically reduced flare and ghosting effects.
The silent wave motor (SWM) is responsible for fast and quiet autofocusing. It has built-in image stabilization. (Nikon's VR II)
This Nikon is a mid-range (85mm) telephoto lens with ultra-fast f/1.4 aperture. It delivers brilliantly sharp images and beautifuly out-of-focus backgrounds, which makes it great for portrait and wedding photography.
Thanks to its aperture of f/1.4 it is superb in low-light conditions, and Nikon's nano crystal coating prevents any flare or ghosting to affect image quality.
At 100mm this is one of Canon's best mid-range telephoto lenses. It boasts a fast f/2.8 aperture and a sophisticated hybrid image stabilization. The IS deals with both shift and angular camera shake, which means even higher image clarity.
Thanks to Canon's USM technology, autofocus ist predictably fast and silent.