Sunlight streaming in can be a great way to break up an interior. It can brighten up the room, or an area thereof, and make it more inviting. Sunlight can even direct the viewer’s eye throughout the scene. However, relying on direct sunlight for interior photography can wreck an image.
Of course this is not always the case, but many times, it is better to make your own sunlight.
First it's reliability. Having a cloud free sky 24/7 would be great, but, where I live and work, it isn’t going to happen. To make due, I create my own sunlight from time to time, which can be quite effective. The image above was captured with a full overcast using a Profoto strobe and Magnum reflector outside.
A second reason, I can control the power of my "sunlight." Direct sunlight just dominates the image; it is so bright that almost none of the ambient interior light will affect the image. Trying to overcome this with supplemental fill and/or HDR layering can actually make the image look flat. By using indirect sunlight, I can use a longer exposure, filling the image with ambient light, and add a splash of “sunlight” as strong as I need it, like in the image to the left
Third, the sun is always moving; its position and angularity change throughout the day and year. If you are a little behind, the sun could have past by where you need it for the next image or, worse, it can create a distraction. Also, it could just never get there depending on the time of year. Imitating it is the only way to ensure it will be where you need it. In the image below, the sun would never enter the room like that.
Last, not all sunlight is created equal. On a clear day, sunlight will be harsh and direct, but with some clouds it may be soft and diffused. Manufacturing sunlight gives me the option of picking what type of sunlight I need, regardless.
Of course using real sunlight does have its advantages especially during the golden hour. Sunlight produces the harshest shadows, it does not dim down the further it projects into a room, and it always produces parallel shadows, all of which are very hard, and expensive, to imitate.