Portrait photography is certainly one of the most popular disciplines, and has been for many years. It's easy to see why - people can easily form a positive association and attachment to a photograph of other people, especially if it includes them or their loved ones. It invokes memories and emotions than other disciplines just can't match. It also affords the photographer a great variety in what they can create given a suitable location and willing model!
First however we must consider some other controllable aspects of the situation. As the subject is now relatively small (compared to landscapes or buildings) we now have some say in the lighting and immediate vicinity. Moving the model or rotating around them brings different backdrops into play, and alters the light quality and direction (if using natural light only). You can also now introduce artificial lighting to add to or completely change the ambient feeling of the location. Really this gives you a huge range of looks from seemingly the same location. You can be incredibly creative.
Traditionally, good portraits focused on the person (the subject) and not on any other surrounding or distracting elements (the foreground or background). This meant a wide aperture to give a shallow depth of field. Only the subject, or even just key parts like the eyes, would be in focus while the background would melt away into creamy blurriness. This idea of subject isolation is still very much key today, with perhaps use of framing or leading lines to emphasise the model more, but keeping them as the focus of the photograph. If you want to further emphasise the face and upper body, typically you would use a longer focal length to give a tight crop. We're talking around the 85mm - 200mm mark. There are a large number of lenses that fall within this focal range so you do have a choice of lenses to use. As mentioned in my zooms vs prime blog, primes will typically afford a wider maximum aperture and be smaller and lighter than their zoom counterparts, but will require you to physically move to 'zoom' in and out. This is normally more favourable for portraits in most situations and has given photographers incredible results for years, but a zoom will give more versatility and make the process a little quicker.
The longer focal lengths (heading into the 100mm and up range) will also introduce an effect called compression. When you are photographing into the distance, with your subject somewhere between you and a background that stretches off behind them, at a high focal length the captured photograph almost appears to show the background a lot closer behind the subject than it actually is. It brings the background elements more into the middle of the image. This is often a pleasing effect and look, but not always. It also tends to flatter people a little more by 'flattening out' their features slightly.
If you ask a photographer that has been specialising in portraits for many years, they may well say the best focal length is about 85mm (the wider the aperture, typically the better! Canon do an f/1.2 lens at 85mm, with good offerings from Nikon and Sigma too). This is still a good benchmark and no doubt you will get some great results, although the depth of field as such wide apertures is incredibly thin at times! Another route to go is to go even longer with the focal length. Looking now to 100mm, 135mm or at the long end of a 70-200mm lens. Remember though if you do use a zoom, you only get that compression by using it at 200mm, rather than at 70mm and moving closer to the subject. They will have the same 'size' in the image but the effects will not be there!
So traditional portraits were very much about the model, the human element, and not the surroundings. Now there is such a thing as an environmental portrait. This encompasses the surroundings to a much greater extent than a tight crop of the head and shoulders. It adds much more context to the photograph and is very useful for business portraits where you want to showcase the working environment around the person, or for weddings where you want to include guests around the couple, or the chaos of the bride getting ready. There are seemingly many applications for wanting a wider focal length for portraits, and its something I often go back and forth between for my work!
As has been the case so far, there isn't a really a right or wrong answer when it comes to the ideal lens for portraits. It all depends on the look and feel you are trying to achieve. If you want to incorporate the surrounding environment then go for something like a 35mm lens. If you want a more traditional tight crop then look at the longer focal lengths, 85mm and above, with the higher end giving you an element of compression as well. I would tend to use a prime lens given the chance, as the minimal bulk ultimately makes it a more pleasurable experience.
Informative posts about weddings and related things, as well as general photography stuff.