Photography is all about light and the manipulation of light. While you can create great photography without understanding the how and why of light, a better understanding can give you more confidence and control.
What is Light?
Light is a part of the electromagnetic spectrum, which includes radio waves, gamma rays, and ultraviolet light. It is completely weightless and made entirely of energy. As you can deduce from the name, light is part magnetic and part electrical.
Light travels in bundles that are called photons. As it travels, the energy produces an electromagnetic field around the photons. The electric and magnetic fields constantly fluctuates from positive to negative and back. Whenever the magnetic field is at its maximum strength, the electric field is at its minimum. The opposing charges make the overall charge a constant zero.
The image above shows one half of the electromagnetic field. As the photon travels, the field fluctuates. The magnetic and electric fields are identical except that they are perpendicular to each other.
Although all photons travel at the same speed, the rate at which the field fluctuates varies. The more energy the photon has, the faster it fluctuates. The rate of fluctuation is called the frequency and is what produces visible color. We measure frequency in a scale of Hertz. Hertz is the number of wavelengths that pass a point in one second. A more common measurement is megahertz (1,000,000 Hertz).
Red light has the slowest frequency while green light waves have a slightly higher frequency. Blue and violet light have the most energy and, therefore, have the highest frequency.
Properties of Light
There are several qualities of light that can be used to describe it. The three most important qualities for photography are brightness, contrast and color. In order to judge or describe different qualities of light, we need to be able to define them.
Brightness is probably the most important of the three qualities. Without enough light, images can’t even be captured. Brighter light allows the use of lower ISOs and larger apertures for better quality images and more control over depth-of-field.
Normally, a brighter light source is better, but sometimes a dimmer light source can be desirable if it also produces better color or contrast.
Light sources can crate high or low contrast light. High contrast light hits an object from a single angle, and is usually created by small light sources. Low contrast light hits an object from many different angles and is usually created by large diffuse light sources.
High contrast light creates hard shadows with sharp defined edges. Light that is low contrast and more diffuse creates softer shadows with edges that are much less defined.
A light source’s size isn’t solely determined only by its physical size. The distance between the light source and an object also plays a part in the relative size. A large light source can become a small, high contrast source when it is far enough away from an object. The sun, for example, would be a huge light source if it were closer, but it’s so far away that it is relatively small and produces hard shadows. On days that are overcast, the clouds diffuse and scatter light turning the sun into a large low contrast light source.
Photographs can be created using any of the visible colors of light. Most modern cameras can capture a wider frequency of light than visible by the human eye. This is why UV light can sometimes degrade image quality. Older film cameras required using a UV filter to reduce the effect, while newer DSLRs have a UV filter built into the camera.
In most photographs, a balanced white light is desired. Ideally, white light has an equal balance of red, blue, and green light. Cameras can detect much smaller changes in color temperature than the human eye. Our eyes constantly adjust to differences in color temperature, and unless the lights are directly next to each other, it can be very hard to discern differences. The white balance setting on your camera adjusts for differences in color temperatures to make the image appear balanced in color.
Color temperatures are measured in the Kelvin scale. A high color temperature gives off a cool bluish light. Light with low color temperatures appears red or orange and is called a warm light.
Interactions with Light
An identical light that strikes two different objects can appear very different. Different materials and surfaces interact differently with light.
An object can alter light in three different ways: transmission, reflection, and absorption. Materials almost always produce a combination of the three properties. In order to understand light better, we must understand how these interactions affect light.
Certain materials allow light to be transmitted through them. Glass, some plastics, and clean air are examples. Objects that are completely transparent cannot be photographed, because they would be invisible.
When light is transmitted, it is often refracted. The only time that refraction does not occur is when the light strikes the object at a perpendicular angle. At any other angle refraction will occur.
When the light passes through an object, it slows down and causes the ray of light to bend. The light then bends a second time as it exits the object and speeds up again. Different materials refract light more than others.
Transmission can occur in two different ways: direct and diffuse transmissions. Direct transmission is similar to the example above using glass where the light is bent in a predictable path.
Diffuse transmission is much less predictable. Objects that create a diffuse transmission scatter the light in random patterns and diffusing it. Objects that create a diffuse transmission are called translucent. An example of a translucent material would be paper, or white glass.
Translucent materials are used in softboxes and diffusion panels because of their ability to diffuse and scatter light. Placing a diffuse translucent material in front of a light source scatters the light, effectively making it a larger light source.
Reflection is simply light striking an object and being partially or completely bounced off. Reflection is what allows us to see and photograph objects. We don’t actually “see” objects, but rather we see the reflected light bouncing off objects.
Light that has been absorbed can’t be seen or photographed. Absorbed light is emitted as an invisible form of energy, most commonly as heat.