—Christopher Robinson, Editor
Photographic lighting is primarily an exercise in reflection management. Understanding and managing reflection, for the result the photographer wants, is good lighting. We will look at how subjects reflect light and how to capitalize on those reflections.
Types Of Reflection
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. For now, we do not care what type of light source might be producing any of the following examples. Only the reflecting surface matters. Any sort of light, strobe, tungsten or fluorescent could work.
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. Figure 1 shows a diffuse reflection. In it we see light falling on a small white card. Three people are pointing their cameras at it.
If each of these individuals were to photograph the white card, each of their pictures would record the subject as the same brightness. On film, the image of the card would have the same density in each negative. Neither the angle of illumination of the light source nor the camera’s angle of view would affect the brightness of the subject in such a picture.
Other than in lighting textbooks, no surfaces reflect light in a perfectly diffuse manner. However, white paper approximates such a surface. Now look at Figure 2. Notice that the scene contains a mostly white piece of sheet music.
There is a reason that we chose to put the white newsprint in this particular example. All white things produce a great deal of diffuse reflection. We know this because they appear white regardless of the angle from which we view them. (Walk around the room you are in now. Look at the white objects and the black objects from different angles. Notice that the apparent brightness of the black objects may change with viewpoint, but the white objects stay about the same.)
The contrast of the light source does not affect the appearance of a diffuse reflection. It is worth proving this with one more picture of the same scene. The earlier photograph was lit by a small light. We could see that by the hard shadows cast by the objects in it. Now look at Figure 3 to see what happens when we use a large light instead.
Predictably, the large light source has softened the shadows in the scene, but notice that the highlights on the paper look about the same. The diffuse reflection from the surface of the paper is identical to that in Figure 2.
So we now have seen that neither the angle nor the size of the light source affects the appearance of a diffuse reflection. However, the distance from the light to the surface of the subject does matter. The closer the light gets to the subject, the brighter the subject becomes and, at a given exposure setting, the lighter the subject appears in the finished picture.
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. Similarly, a light will have nine times the intensity of the same light moved three times as far from the subject. As the intensity of the light falling on the subject varies, so does that of the diffuse reflection.
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. Intuitively, this seems immediately obvious. Why even bother to mention it? Because such intuition is often misleading. Some subjects, as we shall soon see, do not produce brighter reflections as the light moves closer to them.
Direct reflections are a mirror image of the light source that produces them. They are also called specular reflections. Figure 4 is similar to Figure 1, but this time we have replaced the white card with a small mirror. Both the light source and the observers are in the same positions as they were earlier.
Notice what happens. This time one of the three cameras now sees a blindingly bright reflection, while the others see no reflection at all in the mirror. This diagram illustrates the direct reflection produced when a light is directed at a polished surface such as glass. 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.
So, with all that in mind, it is easy to see why the three cameras see such a difference in the brightness of the mirror. Those positioned on each side receive no reflected light rays. From their viewpoint, the mirror appears black. None of the rays from the light source is reflected in their direction because they are not viewing the mirror from the one (and only) angle in which the direct reflection of the light source can happen. However, the camera that is directly in line with the reflection sees a spot in the mirror as bright as the light source itself. This is because the angle from its position to the glass surface is the same as the angle from the light source to the glass surface. Again, no real subject produces a perfect direct reflection. Brightly polished metal, water or glass may nearly do so, however.
Breaking The Inverse Square Law?
Did it alarm you to read that the camera that sees the direct reflection will record an image “as bright as the light source”? How do we know how bright the direct reflection will be if we do not even know how far away the light source is? We do not need to know how far away the source is. The brightness of the image of a direct reflection is the same regardless of the distance from the source. This principle seems to stand in flagrant defiance of the inverse square law, but an easy experiment will show why it does not.
You can prove this to yourself, if you like, by positioning a mirror so that you can see a lamp reflected in it. If you move the mirror closer to the lamp, it will be apparent to your eye that the brightness of the lamp remains constant. Notice, however, that the size of the reflection of the lamp does change. This change in size keeps the inverse square law from being violated. If we move the lamp to half the distance, the mirror will reflect four times as much light, just as the inverse square law predicts, but the image of the reflection covers four times the area. So that image still has the same brightness in the picture. As a concrete analogy, if we spread four times the butter on a piece of bread of four times the area, the thickness of the layer of butter stays the same.
Figure 5 has a mirror instead of the earlier newspaper. Here we see two indications that the light source is small. Once again, the shadows are hard. Also, we can tell that the source is small because we can see it reflected in the mirror. Because the image of the light source is visible, we can easily anticipate the effect of an increase in the size of the light. This allows us to plan the size of the highlights on polished surfaces.
Now look at Figure 6. Once again, the large, low-contrast light source produces softer shadows. The picture is more pleasing, but that is not the important aspect. More important is the fact that the reflected image of the large light source completely fills the mirror. In other words, the larger light source fills the family of angles that causes direct reflection. This family of angles is one of the most useful concepts in photographic lighting. We will discuss that family in detail.
The Family Of Angles
family of angles that produces direct reflection. In theory, we could also talk about the family of angles that produces diffuse reflection. However, such an idea would be meaningless because diffuse reflection can come from a light source at any angle. Therefore, when we use the phrase family of angles, we will always mean those angles that produce direct reflection.
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. Figure 7 shows the effect of lights located both inside and outside this family of angles. As you can see from Figure 7, any light positioned within the family of angles will produce a direct reflection.
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.
Applying The Theory
Excellent recording of a subject requires more than focusing the camera properly and exposing the picture accurately. The subject and the light have a relationship with each other. In a good photograph, the light is appropriate to the subject and the subject is appropriate to the light. The meaning of appropriate is the creative decision of the photographer. Any decision the photographer makes is likely to be appropriate if it is guided by understanding and awareness of how the subject and the light together produce an image.
Light: Science & Magic by Fil Hunter, Steven Biver and Paul Fuqua is available at Amazon and other retailers.
|Photographers diffuse the light source, whether it’s a strobe or a continuous source by reflecting the light from an umbrella or by covering it with a translucent material. We call light passing through translucent material diffuse transmission. Now we speak of diffuse reflection. The two concepts have enough in common that we should pay special attention to the differences between them.
Diffusing the light source has no effect on whether the reflection is diffuse. Remember that small light sources are always “hard” (undiffused) and that large light sources are almost always “soft” (diffused). Strobes, for instance, are a classic hard source until you modify them with a softbox or other modifier.
Then notice that our figures show diffuse reflections produced by both diffused and undiffused light sources. Similarly, you can see direct reflections produced by diffused and undiffused light sources.