Just a list of guidelines that I feel can add to any portrait. Of course, all rules are made to be broken and photography is an art, not a science. Feel free to bend or break any guideline here. Most of these were found by doing simple search on the net and many are ones that I've come to to use to improve my portraits.
1 - The Rule of thirds. By dividing up your composition with 2 horizontal and 2 vertical lines all evenly spaced, you create a grid. Placing the center of your subject in any area where a horizontal and vertical line meet is esthetically more pleasing.
2 - If the subject is placed to one side or the other within the frame of your composition, it is esthetically more pleasing if they are looking "into" the frame more than if they are looking "out of" the frame.
3 - Try to never shoot a subject's head and shoulders "straight on". Angle them to the right or the left a little. A 90-degree shot is too much. Something between 15-60 degrees gives better results.
4 - Eyes: For that "thoughtful" look, have the subject look off to about 15 feet behind and 2-3 feet to the side of the camera. Their head is turned into the position of the leading shoulder. For example, if the left shoulder is closer to the camera, their heads are turned slightly to the left.
4a - For achieving that feel of "direct contact" with the person viewing the picture, have the subject look directly into the camera.
4b - Proper Eye Direction-Generally speaking, in males the eyes should follow the direction of the nose. For females there should be slightly more whites of the eyes on one side than on the other.
4c - The eyes should never be turned so far in their sockets that there is no white area on one side, and you should not shoot into the whites of the eyes. A slightly higher camera angle will give more whites at the bottom than at the top which is more appealing in images of young females.
4d - The catchlights from the main light are considered correctly placed when they appear at the eleven o'clock or one o'clock position in both eyes. One catchlight is normally preferred because there is only one sun but two or more is acceptable. One catchlight per eye suggests thoughtfulness; more than one creates a look a merriment. In multiple catchlight images the catchlight from the main light should be the brightest.
6 - Every subject needs to portray a sense of emotion, but for heaven's sake that emotion should not be boredom. Guide the subject to express the emotion you need. Ask them to convey rather:
- ... anything but looking bored! (ok bored could be shot too... lol)
7 - Depth of field in a portrait is VERY important for two reasons. First, a picture of a face where the eyes are sharp and the nose and ears are blurred is a FAILURE as a portrait shot. We want the complete subject to be sharp. Second, every lens has it's sweet spot where it takes pictures at it's sharpest. If you don't know where that is on your lens... its time to find out, and USE that setting or a setting as close to it as possible to get your pictures TACK sharp.
8 - While composing, always try to shoot a little bigger to avoid "amputating" body parts by accident. You can always crop the picture during post process to frame the picture as you desire. When you do crop, do so between the joints, not on them. Cropping at a joint makes the subject appear amputated. Also when cropping leave room for the subject to breathe (room in front of their face) and leave room for the subject to think (don't severely crop off the top of their head.) Generally if you crop off the top of the head you cannot also crop off very much in front of their face at the same time. If you do they will look crowded in the frame.
9 - In head/shoulders shot, the only thing showing skin should be the head and perhaps the shoulders. Short sleeves are distracting and take away from the picture. While we are at it, for a portrait session, let's leave the shorts at home, ok?
10 - Avoid clothing that is highly bright or has bold patterns. This distracts from the face, which should be the center of attention in a portrait.
11 - Sit or stand tall. No slumping shoulders, no bad posture.
12 - Men lean slightly into the camera. Ladies lean slightly away.
13 - The head should be slightly in front of the belt line.
14 - Ladies should tilt the head towards the higher shoulder. Men tilt towards the lower shoulder. The tilt of a head towards the higher shoulder gives an air of being more feminine. Though the ladies really can get away with a head tilt in any direction, a man looks very feminine if he tilts the head towards the higher shoulder.
15 - Proper Camera Height - Generally speaking, the camera lens should be at about eye level for head and shoulders portraits, chin level to chest level for ¾ length and chest level to waist level for full length portraits. An even lower camera height for heavy set brides, that are posed standing, will make her appear taller and more "regal."
16 - Avoid Flat Lighting. Portraits usually look best with one side brighter than the other side. When the shadow side of the face is closest to the lens it is called short lighting. Short lighting will make the face appear narrower and more slender. If the shadow is on the side opposite of the lens it is called broad lighting. Broad lighting will make the face appear wider and heavier. Flat lighting is where there are no shadows on the face at all.
17 - Watch the Nose and Cheek Line-The face should not be turned so far away from the camera that the nose will break or nearly break the far cheek line. This will make the nose look large.
17a - The far eye should either been seen completely or not be seen at all, but one should never photograph the face so that only 1/2 of the far eye shows. Note that this pose will also cause the subject's eyes to have too much white on one side of the eye as opposed to the other side of the eye.
18 - Avoid Clutter - Simplicity is usually best. *For example too many props in one image. A background that is too busy, or in sharp focus. Another one is a brightly lit background, or a background with large areas of bright sunshine or patches of white sky. These things will attract the eye to them rather than to the subject.
19 - Busy clothing will create confusion for the eye. The eye should immediately be drawn and rest upon the subject. Using a telephoto lens with a large aperture will give you short depth of field which will blur the background.
20 - Watch recessed cheek line with glasses. Have the subject obtain empty eyeglass frames which will solve the problem immediately. Another suggestion is to shoot at least one image without the glasses which will give you a good set of eyes. You can then in Photoshop add them to the image that the subject has chosen to purchase. This technique will also take care of the distortion caused by eyeglass lens refraction which makes the subjects eyes look larger or smaller than what they really are. Angle the head in such a way that unwanted reflections are minimal or completely gone.
21 - Don’t overuse hair and kicker lights-The hair light should kiss the hair, not blast it. Same for the kicker lights.
22 - If it bends, bend it. This rule is perhaps the single most important rule.
22 a - Wherever there is a joint, “break” it. Combining these rules along with "Lean the body over the belt buckle", "Lean slightly away from the camera", "If the subject has two of them make them different" and "Avoid 90 Degree Arms", will create a very dynamic image devoid of any staticness. These rules are basically true whether the pose is a head and shoulders pose or a full length seated pose.
23 - Don’t stack the hands or clasp them together. Seperate them and place them apart and between the joints. Hide the rear hand if possible in group portraits.
23a. Don't place an elbow directly on top of the knee. Place the lower arm area that is 1/2 way between the elbow and wrist 1/2 way between the hip and knee. That way the elbow does not end at the knee forming an uninteresting straight line.
24 - Subdue the Near hand-The hand that is nearest to the lens will appear larger than the other hand especially if it is held very close to the lens (foreshortening.) Feet or legs pointed toward the camera will also look elongated due to foreshortening.
25 - Don't project the hands toward the lens and keep them within the range of focus.
25a - Generally speaking, if the hand is above the subject's waist, the fingers should be directed upwards. If below the waist, the fingers should be directed down. The little finger side of the hand photograph's best.
26 - Don’t photograph the back of women's hands. The sides are much more graceful than the flat of the hand. Fists are masculine, open hands are feminine.
27 - Place the weight on the back foot- and shift the hips (with standing poses.) This will put the hips and shoulders at a pleasing angle. Ask the subject to point her foot that is nearest to the camera toward the camera and shift her weight to the back hip. This means the front leg and foot will have no weight on it. She should then bend her knee slightly toward the other leg. This will give a pleasing “S” curve to her body.
28 - In couples portraiture, do not photograph two heads at the same height- Ideally the eyes of the shorter subject (usually the female) should be at about the same height as the mouth of the taller subject.
29 - In couples or group portraiture, have no head directly above another. All heads should not only be at different heights but also not directly above (or below) another subject. The eyes of the subject that is lowest in the group should be at about the same level as the chin of the next highest subject. You may have to have someone take off their shoes or stand on something to get this effect. Lots of photographers use old hard bound books with the covers taped together with gaffers tape for this very purpose. One book will raise someone up about 1 inch, 2 books will raise him or her 2 inches.
30 - Avoid Crotch Shots-Raising the leg that is closest to the camera in a standing pose will prevent shooting into the subject’s crotch. In groups, turning the subject's body to a 45 degree angle and keeping the legs together should prevent shooting into the crotch.
31 - Use the right key and good taste. Generally speaking, a subject dressed in all white looks best in a medium or high key background. Conversely, a subject dressed in dark clothing looks best on a medium or dark (low key) background.
32 - If the subject has two of them, make them different. Feet, knees, arms, hands and elbows at different levels are more interesting than when placed side by side. Examples would be hips are not level, shoulders not level, head is tipped to the proper shoulder, perhaps leaning slightly, his knees, feet, arms, hands and elbows are not side by side but are placed at different and multiple levels. Sameness leads to boringness in a photographic image. After you "rough in" the pose you will most likely want to refine certain things. When refining a pose, do the least important areas first then as you progress toward the actual capture make the final refinements on the most important areas last.
33 - Avoid "90 degree" arms (a general rule for females.) Gently flowing lines usually look best in portraits of women.
34 - Don't shoot into a bare armpit. Either cover it with something or raise the opposite arm and bring the near arm down. I've seen bare armpits covered with the young lady's long hair, flowers, strips of cloth, and small props. With long sleeved clothing shooting into an armpit is usually not a problem.
35 - (Group Portraiture) Have the subjects at either end of the image face in toward the center. This will keep the viewer's attention on the subjects. The master painters of the Rennaissance used this technique to keep the viewers attention within the painting.
36 - Use a telephoto lens. Short telephoto lenses will prevent foreshortening, which is where objects nearest the lens will appear larger than objects farther away. Use of a short telephoto lens in group portraiture will make all the faces approximately the same size regardless of whether they are in the front row or the rear row.
37 - Avoid posing a bride kneeling on the floor. The wedding gown is designed to look correct and usually flows beautifully and therefore photographs best when the bride is standing. A bride seated on a posing stool for a head and shoulders image or for a waist up or 3/4 length shot is of course perfectly acceptable.
Most of the rules I found in THIS post and credit should go to this gentleman.