Portraits can be some of the most fun and enjoyable subjects to shoot in photography, but it is best accomplished with some specific lenses.  The lens you’ll need will, in some ways, be determined by the DSLR body you’re shooting.

In traditional, old school ways of photography, portrait lenses on 35mm film cameras were typically 85mm to about 135mm.  Furthermore, these lenses were also primarily prime lenses, or having a fixed focal length.  At these lengths, the subject is far enough away from you to allow working room, and / or studio lights to be placed without feeling like you’re on top of the subject or so far away you need to scream directions.

Most modern Digital SLR camera bodies have a cropped sensor which effectively multiplies the focal length, 1.5 – 2 times that of the lens, depending on manufacture.  This means if you own a 100mm lens and shoot a Canon with a cropped sensor, your lens is effectively 160mm long.  This is great for sports shooters but not always great for portrait artists.

A few cameras do have full-frame sensors in them, meaning they provide the same focal length as their film counterparts.  100mm lens is a 100mm lens.

Whichever body you shoot, having the final focal length that falls roughly in the 85-130mm range will suite you wonderfully.

In portrait photography, the lens really matters.  Most prefer to shoot fast lenses, that is a lens with a very small aperture number, like f/2.8, f/2.0 or on some lenses, f/1.2.  Shooting these lenses, especially wide open, will create a very small depth of field and bokeh, which helps separate the foreground from the background.  In portraiture, this is key.  The lenses are sharp too, razor sharp!

So, what is all the talk about?  Take a look at this shot on a Nikon D300 (1.5x crop) wit a 50mm f/1.8 lens (75mm effective focal length).

While it’s not a traditional portrait, it shows you how it creates a nice separation for the subject and the city in the background.

Going one step further, the following shot is taken on a 5d mkII (full frame sensor, no crop) with a 100-400 f/4-5.6 lens, at 400mm.

In this portrait of the boy, you’ll see the background is not recognizable, at all.  This is accomplished by the huge shallow depth of field the lens and focal distance provides.

So what lens is perfect?  That really depends on your current camera body and how and where you shoot.  Indoor portraits will be restricted by the size of the room you’re in and shooting at 400mm will be nearly impossible.

While I only have first hand experience with the Canon system, my guidelines and suggestions can be applied across the board.

The Canon 85mm f/1.2 is perhaps one of the more expensive prime lenses made and is absolutely amazing on a full frame body or cropped sensor but if you want to save some money, the 85mm f/1.8 is a great value.  Canon also offers a 100mm f/2 which is very affordable and 135mm f/2, but I’ve found that on cropped sensor bodies they tend to be a little bit long.  For the money, I’ve been extremely happy with a 70-200 f/4L IS lens and even though it’s not a prime, it’s versatile enough to provide me with wonderful images and great bokeh at 200mm, and will work well on either body.