Understanding light: 3 key principles of light

Three key principles of light:

  1. The effective size of the light source is the single most important decision in lighting a photograph. It determines what types of shadows are produced and may affect the type of reflection.
  2. Three types of reflections are possible from any surface: diffuse reflection, direct reflection, or glare. They determine why any surface looks the way it does.
  3. Some of these reflections occur only if light strikes the surface from within a limited family of angles. After we decide what type of reflection is important, the family of angles determines where the light should or should not be.

These three principles are statements of physical laws that have not changed since the beginning of the universe. They have nothing to do with style, taste, or fad. Understanding these principles enables us to decide what lights need to be where before we begin to place them.

Light: The raw material of photography

Photography is the manipulation of light (finding, creating, or modifying light elements).

As photographers, we are primarily concerned with the brightness, color, and contrast of the light.

  • Brightness: 
    • To a photographer, the brightness of a light source is the most important quality of a light source is its brightness. A brighter light is almost always a  better light.
    • At the most basic level, if the light is not bright enough, we cannot get a picture. If the light is brighter than the minimum we must have, then we can probably get a better picture.
    • Usually photographers prefer a dimmer light source only when there is an aesthetic improvement in one of the other qualities of light: the color or the contrast.

 

  • Color:
    • We can use light of any color we please, and very strongly colored lights frequently make an artistic contribution to the photograph. Nevertheless, most pictures are made with white light. However, even “white” light comes in a range of colors. Photographers consider light to be “white” when it is a roughly even mix of the three primary colors: red, blue, and green. Human beings perceive this combination of light colors to be colorless.
    • The proportions of the color mixture may vary to a great extent, and people still cannot perceive any difference, unless they have the different light sources side by side for comparison. The eye can detect a very slight change in the color mixture, but the brain refuses to admit the difference. As long as there is a reasonable amount of each primary color, the brain says, “This light is white.”
    • Digital cameras make the same automatic adjustment to color that the brain does, but not nearly as reliably. Photographers must therefore pay attention to the differences between various white light sources. To classify variations in the color of white light, photographers borrow the color temperature scale from physicists. The color temperature scale is based on the fact that if we heat a material in a vacuum hot enough, it will glow. The color of this glow depends on how much we heat the material. We measure color temperature in degrees on the Kelvin temperature scale. The measurement unit, degrees Kelvin, is simply abbreviated “K.”
    • It is interesting that light with a high color temperature is composed of a disproportionate amount of those colors artists call cool. For example, 10,000°K light has a great deal of blue in it. Similarly, what physicists tell us is a low temperature source has much of those colors artists call warm. Thus, a 2000°K light tends toward the red to yellow family of colors.
    • Some basic K light range: 5500: daylight; 3200: tungsten.

 

  • Contrast:
    • A light source has high contrast if its rays all strike the subject from nearly the  same angle. Light rays from a low-contrast source strike the subject from many different angles. (Sunlight on a clear day is a common example of a high-contrast light source; while on an overcast day, sunlight becomes a low-contrast light source.)
    • The easiest way to recognize a high-contrast light source is the appearance of the shadows. A shadow with sharply defined edges is called a hard shadow. For
      this reason, high-contrast light sources are also said to be hard lights.
    • In the photograph using low-contrast light, the shadow of the subject is no longer clearly defined. It is no longer hard. The viewer cannot decide exactly what part of the ground (that the subject stands) is in shadow and what is not. A shadow such as this one, with no clearly defined edge, is called a soft shadow, and the light producing it is called a soft light.
    • Notice that we are using the words hard and soft only to describe how sharply the edge of a shadow is defined. We are not using these terms to describe how light or dark the shadow is.
    • For single light sources, the size of that source is the primary factor influencing its contrast. A small light source is always a hard light source, and most large sources are soft ones.
    • Notice that the physical size of a light does not completely determine its effective size as a photographic light source. We know that the sun is more than 1 million kilometers in diameter. However, it is far enough away to act as a small source for a photographic subject on Earth. If we could move the sun close enough to us, it would become an extremely large light source. We could then make softly lit photographs in sunlight, even without any cloud cover, assuming we could find a solution to the heat problem.
    • Note: The Contrast of a Photograph:
      • The contrast of the light is only one of the influences on the contrast of a photograph. If you are an experienced photographer, you know that you can find high contrast in an image with low-contrast light and vice versa.
      • Contrast is also determined by subject matter composition, exposure, and development. As everyone knows, a scene that includes black and white subjects is likely to have more contrast than one with entirely gray objects; but a software Levels or Curves adjustment can produce high contrast, even in an entirely gray scene in very-low-contrast lighting.
      • The relationship between exposure and contrast is a bit more complex.  Increased and decreased exposure can reduce contrast in an average scene. However, increasing exposure will increase contrast in a dark subject, whereas decreasing exposure may increase contrast in a light
        gray scene.

 

Light vs lighting

  • Shadow is the part of the scene that the light does not strike. Highlight is the area illuminated. The two below photographs have very different lighting, and you can see a difference in the highlights in the two pictures. However, the difference in the two highlights is minor; most viewers will notice only the difference in the shadow.
  • Photographic lighting is more than just light. Lighting is a relationship between the light, the subject, and the viewer. If we want to say any more about lighting, we must talk about the subject.

understanding_light_001_two-pepers

 

How the subject affect lighting:

  • Light photons move. Photographic subjects often sit still. This is why we tend to consider light to be the “active” player in the photographic event. But this attitude handicaps our ability to “see” a scene.
  • Two identical photons striking two different surfaces can appear dramatically different to the eye and to the camera. The subject changes the light, and different subjects change the light in different ways. The subject plays an active role, just as the photon does. To perceive or to control lighting, we have to understand how the subject does that.
  • The subject can do three things to a photon that strikes it: it can transmit, absorb, or reflect that photon.
    • Transmission:
      • Light that passes through the subject is said to be transmitted. Clean air and clear glass are examples of common materials that transmit light.
      • Showing you a photograph of transmitted light would be useless. A subject that only transmits the light cannot be seen. The subject that does not alter the light in some way is invisible.
      • Simple transmission: light comes into the subject then go out straightly.
      • Refraction: is the bending of rays of light as they are transmitted from one material to another.
        • Some materials refract light more than others. Air, for example, refracts light very little, whereas the glass used in a camera lens refracts it a great deal.
        • Refraction is caused by a variation in the speed of light caused by the material through which it is transmitted
        • Unlike simple transmission, refraction can be photographed.
        • understanding_light_002_refraction
      • Diffuse Transmission: 
        • So far we have talked about direct transmission, in which light passes through a material in a predictable path. Materials such as white glass and thin paper scatter the light rays in many random, unpredictable directions as they pass through. This is called diffuse transmission.
    • Absorption: 
      • Light that is absorbed by the subject is never again seen as visible light. The absorbed energy still exists, but the subject emits it in an invisible form, usually heat.
      • Like transmission, simple absorption cannot be photographed. It is “visible” only when we compare it to other light in the scene that is not absorbed. This is why highly light-absorbing subjects, such as black velvet or black fur, are among the most difficult things to  photograph.
      • Most subjects absorb part, but not all, of the light striking them. This partial absorption of light is one of the factors that determine whether we see a given subject as black, white, or some intermediate gray.
      • Any particular subject will also absorb some frequencies of light more than others. This selective absorption of certain light frequencies is one of the factors determining the color of a subject.
    • Reflection: 
      • Reflection is light striking a subject and bouncing off.
      • Reflection makes vision possible. We do not see objects; we see light. Because most objects produce no light, their visibility depends entirely on light reflected from them.

 

Management of Reflection and the Family of Angles

Of the three ways the subject can affect the lighting, reflection is the most visible. Highly transparent subjects have minimal effect on light, so they tend to be invisible. Highly absorbent subjects may also be invisible because they convert light into other forms of energy, such as heat, which we cannot see.

Photographic lighting, therefore, is primarily an exercise in reflection management. Understanding and managing reflection, for the result the photographer wants, is good lighting.

Different subjects reflect the light differently, hence create different image in our brain. For example, we can easily see differences among a paper, a metal panel, and a ceramic pile, even they have same color and same size.

Types of reflections

Light can reflect from a subject as diffuse reflection, direct reflection, or glare. Most surfaces cause some of each of these three types. The proportions of each type of reflection vary with the subject, and it is the proportion of each reflection in the mix that makes one surface look different from another.

  • Diffuse Reflection:
    • Diffuse reflections are the same brightness regardless of the angle from which we view them. This is because the light from the sources is reflected equally in all directions by the surface it strikes.
    • The Inverse Square Law:
      • A diffuse reflection gets brighter if we move the light source closer to the subject. If we needed, we could calculate this change in brightness with the inverse square law. The inverse square law says that intensity is inversely proportional to the square of the distance. Thus, a light at any particular distance from the subject will light the subject with an intensity four times as bright as the same light twice as far away.
      • Ignoring the math, this simply means that reflection from a surface gets brighter if we move the light closer and it gets dimmer if we move the light farther away.
  • Direct Reflection:
    • Direct reflections are a mirror image of the light source that produces them. They are also called specular reflections.
    • The light rays bounce from the smooth surface at the same angle at which they hit it. More precisely stated, the angle of incidence equals the angle of reflectance. This means that the point at which direct reflections can be seen is
      exactly determined by the angles between the light source, the subject,
      and the camera viewpoint.

The Family of Angles

In reality, each surface is made up of an infinite number of points. A viewer looking at a surface sees each of the surface’s points at a slightly different angle. Taken together, these different angles make up the family of angles that produces direct reflection.

understanding_light_003_family_of_angle

This family of angles is important to photographers because it determines where we should place our lights. We know that light rays will always reflect from a polished surface, such as metal or glass, at the same angle as that at which they strike it. So we can easily determine where the family of angles is located, relative to the camera and the light source. This allows us to control if and where any direct reflection will appear in our picture.

Any light positioned within the family of angles will produce a direct reflection. A light placed anywhere else will not. Consequently, any light positioned outside of the family of angles will not light a mirror-like subject at all, at least as far as the camera can see.

Photographers sometimes want to see direct reflection from most of the surface of a mirror-like subject. This requires that they use (or find in nature) a light large enough to fill the family of angles. In other scenes, they do not want to see any direct reflection at all on the subject. In those instances, they must place both the camera and the light
so that the light source is not located within the family of angles.

Polarized Direct Reflection

A polarized direct reflection is so similar to an ordinary direct reflection that photographers often treat them as the same. However, these reflections offer photographers several specialized techniques and tools for dealing with them.

Placing a polarizing filter over the light source will turn a direct reflection into polarized reflection.

Polarized light sources are not restricted to studio lighting. The open sky often serves as a beautifully functional polarized light source. Facing the subject from an angle that reflects the most polarized part of the sky can make the lens polarizing filter effective. This is why photographers sometimes find polarizing filters useful on subjects such as bright metal, even though the filter manufacturer may have told them that polarizers have no effect on such subjects. In those cases, the subject is reflecting a polarized source.

(need to get more information)

 

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