How Light Works

How Light Works

by William Lulow

Light is the deciding factor in almost all successful images. Certainly, without it, we would be unable to take pictures (except for xrays and infrared). So, understanding how light works is important to all photographers. After all, the word PHOTOGRAPH actually means “light picture.” So, here’s some science for you:

First, light rays are parallel. They travel in the same direction from their source. That could be the sun as well as a light bulb. Light from a light bulb with no modifier (shade, reflector or diffuser) will travel in random parallel lines in all directions. This is usually of no help to a photographer. Photographs are usually most successful when light rays are DIRECTED, DIFFUSED, or MODIFIED. 

If you are photographing a person, you would need light to be directed at them. This requires the use of a reflector to direct the light. Reflectors can take many forms:

• A round, piece of aluminum or metal that can direct a bulb’s rays
• A large, square box with some diffusion material placed over the front. The light would then be gathered inside the box and aimed through the material to illuminate the subject.
• A small, narrow, cone-shape which will restrict the light rays and aim them at a specific point on the subject.
• A large, round umbrella which gathers light rays and BOUNCES them back toward the subject. There are many different types and sized of umbrellas that give different effects on subjects.
• A shade made of metal or wood or heavy-duty paper can be used to add light to or keep light from a subject.

Here are a few reflectors:

If you are photographing a person and want that person’s features to be soft, you would need a diffuser, something to shine the light through to make it less harsh. Or a large reflector to increase the size of the light. Softness in a portrait comes from having the light source much larger than the subject. Umbrellas serve this purpose quite nicely.

Diffusers are made of different materials and come in different sizes:

• Spun glass – which doesn’t burn and can fit in front of a light with a reflector.
• Clouds – clouds are natural diffusers that soften light from the sun
• Other translucent materials such as white fabric.

Here is an example of a light diffuser:

If you are photographing a scene, you would need light to cover the entire space of the image so that you could see all the detail. If you are outdoors, during daylight, sunlight will take care of this. If you are indoors, where there is little or no light, you will have to add light to the scene. This can be done in several ways:

• Use a light bulb or a number of bulbs to illuminate the scene.
• Use a flash bulb to give a brief but bright light to illuminate the scene.
• Use a number of lights to illuminate the scene.
• Use a long exposure in the camera to let in as much light as possible which will serve to illuminate the scene.

Second, light, when it travels from a light source to a subject, behaves in certain, specific and well documented ways.

• It will travel a certain distance and then get weaker with increased distance. So, it follows that the further away from a light source you are, the weaker (less bright) it will be. The converse is also true. The closer you are to a light source, the brighter the light will be.
• The interaction of light with distance creates one of the constant “laws” of light. It is called the “INVERSE SQUARE LAW.” It states that the intensity (brightness) of a light will vary inversely with the square of the distance between the light and the subject. In other words, if you move a light source twice the distance from a subject, the effect of the light will be half as bright as it was originally. Conversely, if you move a light half the distance to the subject, the effect of the light will be twice as bright.
• Every light source has a given range for its light to fall on a subject. If you extend that range, your subject will be dark. If you decrease that range, your subject will be bright. This is why, in many photographs, backgrounds (objects far away from the main light source) have to be illuminated separately.

Third, natural light comes from the sun which is 92 million miles from Earth. So, because it is so far away, it acts as a “point” light source most of the time. It is very bright and creates very distinct shadows on Earth. For most photographs, we need light that is not as harsh as the sun’s rays so that we can see all the details in what we are trying to capture.  This explains why, for photographs and portraits, we need more diffused light such as that provided by sunlight filtered by clouds. The clouds act as a rather large diffuser.

This, briefly, is why we should think about how to control light. A photographer’s studio is one place where we can practice controlling light for specific photographic purposes.