Both high-key and low-key techniques are used to illuminate a scene / model / object. They tend to serve specific purposes, for example low-key lighting lighting renders a dark and mysterious look to a scene. It can also define shapes, accentuating them. Low-key lighting is also the key to make dynamic portraits. This lighting technique is used to capture sorrow, mystery, low profile, winter and other moods and scenes. On the other hand, high-key lighting is used for creating bright and vibrant images. High-key is synonymous with happiness, peace, fresh beginning, summer etc.
In low-key lighting there is an ample amount of shadow in the scene. This is because normally this technique uses a single light source. If there is an artificial light source it is placed at a distance from the subject, so as to make it hard. At other times the photographer would only shoot with whatever ambient light is available. The main thing that identifies a low-key image is an awful amount of contrast. Contrast is the range of tones between the brightest and the darkest tones in an image and in a low-key image there’s plenty of it.
High-key on the other hand is all about light, washing out shadows from every nook and corner of the frame. As a matter of fact photographers setting up their lights to shoot high-key photographs prefer to use an abundant amount of light; often using three or more. High-key photography is about lesser contrast and an abundance of highlight.
Setting up a high-key shot requires you to use at least three lights. These three lights would be two main lights (known as key lights), and one background light. That being said, the more lights – the better in high-key photography. You may come across high-key images shot with up to five lights, three for the subject and two for the background.
Background Lighting and Subject To Background Distance
One thing to remember about the background is it must compliment the type of lighting used for the photo. The background is just as important as the subject when it comes to great photography. Often photographers get this part wrong and forget to illuminate the background. You need at least one dedicated light to properly illuminate the background and ensure that there is no fall-off.
Low-key photography warrants that you use a dark (preferably black) background. On the other hand for a high-key shot the background must be very bright (preferably white). Subject can be much closer to the background when taking high-key lighting shots than in low-key lighting. However, take care that there is no background light spilling on to the subject’s face.
Low-key photos don’t require the background to be illuminated. Plus, you can ask the model to stand at a distance from the background, so that there is a greater light fall-off. The bigger the light fall-off, the more obscured the background will be and will add more to the low-key look that you are after.