Although artists have used light coming through a window to light their subjects for centuries, the most famous artist to use natural window lighting was the Dutch artist Rembrandt Harmenszoon van Rijn (July 15, 1606 – October 4, 1669). In photography books, when an author refers to Rembrandt lighting, they refer to soft, broad, natural light coming strongly from one side, and leaving the opposite side of the face in shadow. This is not direct sunlight, but rather light from the sky, preferably the northern sky or "north light". This is a beautiful look, and creates mystery in a portrait that makes it intriguing. See http://en.wikipedia.org/wiki/Rembrandt for Rembrandt painting examples.
Direct sunlight creates very "hard" distinct, dark shadows, with a very hard edge going from light to dark. Portraits done in direct sunlight without a reflector or flash fill can look very harsh and have too much contrast. There is no detail in the shadow areas at all. In contrast, sky light coming through a window creates a very soft edge, and as light turns to shadow it is very gradual. This soft look is very flattering, and classic Rembrandt lighting is something that every professional photographer should master. Rembrandt lighting can and should be imitated using artificial lighting, but it can never match natural north light.
North light coming through a window is not very bright compared to direct sunlight. You will need to do several things to compensate for the low light levels.
1. You will need to use a tripod. Often shutter speeds for natural light photography go as low as 1 or 2 seconds. The slowest you can hand hold a camera without visible shake is 1/30 of a second. Using anti-shake or "image stabilizer" lenses can help reduce vibration, but there is no substitute for a steady tripod.
2. Increase your ISO setting. The default ISO for outdoor photography is 100, which on many digital cameras is the lowest setting, and creates the least amount of noise. Digital cameras continue to improve, and each year the cameras allow higher and higher ISO settings, making the camera more sensitive to light, but producing less noise (grain, visible pixels) in the picture. I often use a setting of 400 or 800 ISO with my Canon 5D Mark II, and have no fear of producing a unusable file.
3. Work with a model that is willing to move slowly, and freeze upon command. Many years ago I sat for a portrait for an album cover taken on 50 ISO 4" x 5" film. The only light was coming through a stained glass window in a church. Exposure times ranged from 3 second to 10 seconds! I had to sit without breathing, swaying, blinking, or moving in any way for up to 10 seconds at a time! That length of time is no longer necessary due to the low noise levels in recent digital cameras. However, even 1/2 second is still a very long time to sit still without moving. If you subject blinks during the exposure, their eyes will look blurry.
Tell your model to listen to your count, and to freeze on the count of 3. They should take a half-breath, hold it, and yet have a un-posed, very natural and comfortable expression on their face. This is not an easy thing to do, and more experienced models and actors will often be better at this than first timers.
4. Use a reflector on the shadow side of your model. If what you are shooting is a portrait of the model, you will want to try some of the images with window light only so you can have the dark shadows on one side of their face. However if you are selling something - clothing, props, products, etc, you will want more control over your lighting. Using one or more reflectors can fill the shadows, and make what you are selling easier to see. Reflectors can be white, silver, gold, or zebra, i.e. gold and silver or gold and white, and from 2 feet across for the face only, to 4' x 6' or larger for full body fill. Do not over fill the shadows, or the lighting will be too flat and boring. The light on the window side of your subject should be 2 -4 times brighter than the shadow side.
Photographers often use gold reflectors to "warm up" the photographs, and make it friendlier. This "warming" of the color temperature can also be done in photo editing software such as Adobe PhotoShop, however a gold reflector only warms the shadow side of the person, which is the bluer or cooler side anyway.
5. Use flash fill. I did say fill, not "use flash as your primary, or key light". Using flash that's more powerful than the natural light will give you flat, unnatural looking light. No, you will want the natural light to be 2 -4 times stronger than your flash. Many studio lights do not have power settings low enough to be 2 or 3 stops less than the natural light. If you want to use studio lights for fill, look for ways to reduce the light output, such as by adding neutral density gels, or multiple diffusers, or bouncing off a back wall. However, I have had good result by using my on-camera flash on manual output settings. I put my camera on a tripod, take a light meter reading or take a trial photograph and check the histograms on the back of the camera. Once I feel I have a good exposure setting with the natural light, I add my on camera flash, usually at 1/4 or 1/8 power setting, and bounce the light off the ceiling or a white wall. Do not use colored walls for this unless you want those colors reflecting on your subject!
6. Warm your color temperature up in your photo editing software. Skylight is very blue, often 6,500 to 9,000 Kelvin, compared direct sunlight at noon, a very white 5,000 - 5,500 Kelvin, or direct sunlight at sunrise or sunset, which can be very yellow or orange, at 3,200 - 3,500 Kelvin. Portraits taken in natural light often look blue, or cool. If this is what you are looking for, great, leave it alone. However, many photographers prefer "warmer" tones, more golds, browns and yellows in their portraits. If you shoot your portraits in "RAW format" instead of jpeg, your photo editing software should offer you a sliding scale where you can adjust your color temperature. Click on something pure white in the picture with your white balance tool to get a starting point. Then shift the slider up towards the higher numbers to make the picture "warmer" or more yellow / gold, or towards the lower numbers to make it look "cooler" or more blue. Try color temperature settings of 6,000 - 7,500 and see if you like the warmer look.
7. One of the more popular looks today is with a window appearing in the picture, with the highlights over exposed, or "blown out". Many photo editing books will tell you to set the whitest point in your picture at 230 (on a scale of 0 - 255), that any whiter and there is no detail in the whites. This is an excellent rule for standard commercial photography. However, one current fashion is to break this rule, and allow the windows to go pure white in areas, or totally blown out at 255. A dappled look with tree leaves outside the window going from very light green to white is a favorite of mine. This "high key" look works well with subjects that are dressed in white or soft pastel colors. It looks very soft and romantic, and is great for brides, babies, pretty girls, and couples.
Add out of focus areas, or blur the areas around the model, and you have a very soft, dreamy look. One portrait photographer I know puts a clear or UV filter in front of his lens, and smears a light coat of Vaseline around the outside edge of the filter, leaving the center clear. This creates a soft, blurred vignette around the edge of the picture, and is very flattering to your subject. Of course, many camera companies sell "soft focus" lenses, however none of them are as cheap or as flexible as a jar of Vaseline! Tilt/Shift lenses and "Lensbabies" are also excellent for creating out of focus highlights in natural light portraits.
Natural light is beautiful coming through a window, but you can also get similar effects outdoors. Find a building with a porch, and put your subject just under the overhang of the roof, close to the edge. Buildings with columns or arches are particularly wonderful for outdoor natural light portraits. Have your subject lean against a column or archway. All the same principles apply - use northern skylight, reflectors, flash fill - but you will need to pay more attention to your background. When you become a master at Rembrandt lighting, you will be on your way to becoming a true artist and not just somebody who "snaps pics."