To keep things simple all notes here are based on a "full frame" digital camera (or a 35mm film camera). I do recommend a full-frame camera for portrait work, since it will help get a nice background blur (in combination with the right lens and other setup), however, a camera with a smaller sensor can work also.
For taking a portrait of a single person, you generally are going to want to use a focal length between 50 and 135mm. For a head and upper torso shot, I generally use 85mm. I might use a longer lens if there is some reason I need to be farther away from the subject (e.g they are on stage and I'm in the audience, or they are playing goalie in a soccer game and I'm on the sidelines). (As with pretty much any photography, you want to choose a lens such that you can fill frame with your intended subject matter.) Keep in mind that a long lens (e.g. 135mm or more) will have the effect of making the subject's face look kind of flat (not as three-dimensional). This can be beneficial for minimizing a broad face or large nose, but also can look a bit lifeless. On the other the other end of the spectrum, using a wide angle lens can distort a person's face so that it looks unnaturally wide. For example, I do not recommend using a 35mm lens (or wider) for a head and shoulders shot. Also consider that if for example you use a 50mm lens for a head and shoulder shot, assuming you want to fill the frame, you'll have to be pretty close to your subject and this might make them nervious. An 85mm lens allows you to fill the frame but still maintain a comfortable working distance from your subject. (If you are wanting to get totally candid shots, at a wedding for example, this might be a reason to use an even longer lens, so that your subjects are generally not aware of your presence.)
If you move from a head and sholders shot to a whole body portrait, then you might stick with a 85mm lens if you have room to move farther from the subject, or you might use a 50mm lens instead.
If you are doing a small group portrait, you may need to move to a 50mm lens (rather than 85mm) so that you don't have to be too far away from the subjects.
For a large group portrait, again mainly due to distance constraints, you'll probably want to use a 50mm lense or a even a 35mm lens if you don't have much room to back up. Even though a 35mm lens is wide-angle, it should not distort faces much since you will be farther away, however, do leave some extra room on the sides of the shot, otherwise people on the very edge will be somewhat distorted. Consider that if you have multiple rows of people (some standing in front of others) then you may need to ensure that all rows are in focus.
Whatever focal length of lens you choose, generally having a "fast" lens (one with a fairly wide maximum aperture or low f-number) is desirable for portrait work. Being able to use a wide aperture can help you get a nice background blur, though you don't always need a super-low f-number to accomplish this. But generally for portrait work, a fast lens is desired. For example an f/2 85mm prime lens or an f/2.8 70-200 zoom lens.
As said above, you generally want a lens with a wide maximum apeture, though you don't always need this to achive a nice background blur. Let's look at some numbers (these are from calucations that I won't explain on this page but are detailed on "Distance and focal length calculations" and at the bottom of "Hyperfocal chart and calculator". Note that moving the camera closer to your subject will increase background blur without having to change the aperture setting (assuming the distance between the subject and the background remains the same).
The following charts shows some reaonable portrait setups you could use (this excludes additional important considerations such as distance from subject to background and lighting - we'll get to that next). I've purposely chosen setups where the depth of field is shallow (1 to 2 feet). Normally you want to focus on the subject's eyes - given the depth of field their nose will also be in focus, but starting around the back of their head things will start to go out of focus (the background will be blurred). Note the area captured for each shot is always a 3:2 ratio since I'm assuming a "full frame" sensor (the same size as 35mm film) which has this ratio. For a single-person portrait, normally you'd take the shot with the camera in vertical position, so for example for the "whole body" shot you are using the longer dimension (6 feet in the chart below) for the height of the person.
| lens focal length | distance from subject | area captured | aperture | depth of field |
|---|---|---|---|---|
| 135 mm | 11.25 feet | 3 x 2 feet (head + shoulders) | f/8 | 0.98 feet (10.78 - 11.76 ft) |
| 135 mm | 22.5 feet | 6 x 4 feet (whole body vert. or small group horiz.) | f/4 | 2.00 feet (21.55 - 23.54 ft) |
| 85 mm | 7.08 feet | 3 x 2 feet (head + shoulders) | f/8 | 0.98 feet (6.62 - 7.60 ft) |
| 85 mm | 14.17 feet | 6 x 4 feet (whole body vert. or small group horiz.) | f/2 | 1.00 feet (13.69 - 14.69 ft) |
| 85 mm | 14.17 feet | 6 x 4 feet (whole body vert. or small group horiz.) | f/4 | 2.00 feet (13.24 - 15.24 ft) |
| 50 mm | 4.17 feet | 3 x 2 feet (head + shoulders) | f/8 | 0.99 feet (3.73 - 4.72 ft) |
| 50 mm | 8.33 feet | 6 x 4 feet (whole body vert. or small group horiz.) | f/2 | 1.00 feet (7.86 - 8.86 ft) |
| 50 mm | 8.33 feet | 6 x 4 feet (whole body vert. or small group horiz.) | f/4 | 2.02 feet (7.44 - 9.46 ft) |
| 35 mm | 11.67 feet | 12 x 8 feet (large group) | f/1.4 | 2.86 feet (10.41 - 13.27 ft) |
| 35 mm | 11.67 feet | 12 x 8 feet (large group) | f/2 | 4.15 feet (9.95 - 14.10 ft) |
If you have a solid color background, then it's not as important that the background is blurred, but the more complex your background the more you generally want to blur the background so that a) it does not distract from your subject b) the subject stands out as being sharp against the background. I recommend generally trying to get a blurred background in all situations, because even a solid color background (for example a white wall) might for example have a dent in it which could distract the viewer from the subject if in-focus. (Of course it would be possible to blur such an imperfection in post-processing also, but I think it's best to try to do as much "in-camera" as possible.) So, in order to get a blurred background you need to have some distance between the subject and the background. I think a good rule of thumb is probably that you want the distance between the subject and the background to be at least the same (if not greater than) the distance between the camera and the subject. This is a point that I believe many people do not consider at first. In particular, for indoor photography, this means you need perhaps more space than you realize at first. Taking a row from the chart above, if you are going to shoot your subject with a 50 mm lens from 4.2 feet away at an aperture of f/8, you also want your subject to be at least 4 feet from the background. This means you need at least ~10 feet of space (you also need space for you to stand with the camera or for you to setup a tripod - plus you might want to setup some lighting equipment). If you are going to shoot with an 85mm lens indoors, you are going to need at least 15 feet of space. (So for indoor work, you might prefer a 50mm lens if you do not have much space.)
Of course, light is required, whether shooting indoors or outdoors. For outdoor work, you are more likely to take advantage of natural light, however, in many cases the natural light may not be ideal. For example, harsh, direct sunlight is not good for portrait work. If you have this kind of light you generally want to move your subject into the shade (or put up a barrier to block or diffuse the sunlight) and provide some of your own artifical light to hightlight your subject in the precise manner you desire.
Photographers use essentially 3 types of light. 1) natural light, 2) artifical light in the form of a flash or strobe, 3) artifical light in the form of a continuous light source. All 3 types of light might be diffused, reflected, or manipulated in some way. With artifical light, using a flash or strobe is generally easier because it requires less power, however, a continuous light source allows you to better see how the light affects the scene you are shooting before you take the shot.
You generally want to have only a single dominant source of light, but you may also have less powerful secondary light sources. Human beings are using to having a single sun (being on planet Earth) and so having multiple light sources (that are equally strong) tends to look strange and will make a scene feel unnatural - even one that is indoors.
This could be done outdoors or indoors, but for simplicity let's assume indoors where you control all the lighting, and are using only artifical light. Assume your subject is facing you the photographyer (and your camera) directly, and behind your subject is some type of background. A classic setup is to have your "key" light (the main light source) off to one side or the other so it hits the subject from the side at a 45 degree angle. Furthermore, you position the light above your subject so that it's coming down toward your subject's face at a 45 degree angle (so two 45 degree angles). So you'll need your main light on a stand to get it up high -- also it helps if you have a photograhy studio with a high ceiling. You can still achive a 45 degree downward angle with a regular ceiling but this means the light will have be pretty close to the subject and also pretty close to the ceiling. Having the light in this position looks natural to humans, again because we are using to having a sun up in the sky. However having the light directly above the subject would not be as flattering an angle. Also note that having the light hit the subject straight on is generally not as flattering as the 45 degree sideways angle - the 45 degree angle gives the face more of a three-dimensional look.
Although we wanted the light to come from the side to give more dimension to the face, this does
leave the other side of the face (farther from the light) a bit dark.
Generally we want to give a bit of "fill light" to this other side of the face.
This can be accomplished by either relecting some light from the main light to this side of the face
(using something as simple as large piece of white cardboard),
or using a second but less powerful light - or using a light of the same power but positioning it
farther from the subject so that it is effectively less powerful.
When using a second light, you generally want it just slightly to the opposite side from other light
(so not 45 degrees but almost straight-on), and also not as high as the other light - the
same height as the subject's face is fine. Another technique is to have the main
light pointing down more (a 30-degree angle) and then have a reflector below the subject's face
that will reflect some of the main light back up into the subject to fill in the dark areas.
There are many tutorials online regarding these kinds of setups so I will point out a couple:
5 quick and easy portrait setups (B&H)
(a 5 setup video on YouTube)