A good portrait shows off the subject without distractions.
Portrait photography can be either extremely simple or creatively original. The goal of a portrait is to show off someone's personality without distractions. If you have the budget, you should at least bring two lenses to a shoot to provide some variety.
85mm
An 85mm lens is generally considered the best focal length lens to shoot portraits. This length produces a similar appearance of someone who is standing about 15 feet away, which is similar to what the human mind remembers. This focal length can be accomplished with a prime lens or with an adjustable telephoto. In either case, you'll need to stand farther back from your subject than normal because of the zoom level.
Prime Lenses
Prime lenses are lenses that are unable to zoom in or out. While this seems like a disadvantage, prime lenses are generally sharper and have a greater depth of field, which is the amount of blur in the foreground and background. The increased sharpness brings out the details and clarity that a portrait should have. Major lens manufacturers typically have a 50mm prime lens in their lineup for around $100.
Adjustable Zoom Lenses
An adjustable zoom lens, such as one with a 17mm to 135mm range, can be a convenient lens to have during a shoot. Its extremely variable zoom range makes it easy to frame different parts of a portrait. For example, you may need a wider angle for photographing a torso than simply a face. Instead of changing lenses, you can simply zoom out with an adjustable lens. The disadvantage is that variable lenses typically are slightly lower in image quality than prime lenses.
Fisheye Lenses
In special situations, a fisheye lens can be the appropriate lens for a portrait. These ultra-wide lenses are best suited for more casual, artistic portraits. A portrait of a surfer, for example, may be extremely appealing with a fisheye.
Tags: focal length, prime lens, with adjustable, without distractions