The Angle of Light

What sort of lighting might accomplish this? To answer that question, let us begin by looking at a standard copy setup and at the family of angles that produces direct reflection.

Figure 4.1 shows a standard copy camera arrangement. The camera is on a stand and is aimed at the original art on a copy board beneath it. Assume that the height of the camera is set so that the image of the original art exactly fills the image area.

We have drawn the family of angles from which a light, or lights, can produce direct reflection. Most copy arrangements use a light on each side of the camera. We need only one light to see the principle.

Such a diagram makes it easy to light the setup. Once again, any light within the family of angles will produce direct reflection, and a light located outside that family will not. We also know from Chapter 3 that a light can produce diffuse reflection from any angle. Because we want only diffuse reflection, we place the light anywhere outside the family of angles.

In Figure 4.2 the cigar box is photographed with the light placed outside of the family of angles. We see only diffuse reflection from the surface, and the tone values in the photograph closely approximate the original.

of angles will produce direct reflection; the other will not. There is a similar family of angles on each side of the camera.

setup. The light inside the family

Figure 4.1 The family of angles that produces direct reflections in a "copy" lighting

Lighting Family Angles
Figure 4.2 In a good picture, the box label we see has nothing but diffuse reflections and the tones closely resemble those in the original.

By way of contrast, in Figure 4.3 the light was inside the family of angles. The resulting direct reflection causes an unacceptable "hot spot" on the glossy surface.

This is all straightforward in the studio or the laboratory. However, photographers are also asked to photograph large paintings in museums or other locations from which they cannot be removed. Anyone who has ever done this knows that museum curators always place display cases or pedestals exactly where we want to put the camera. In such situations, we need to place the camera closer to the subject than we might otherwise. We then switch to a wide-angle lens to get the whole subject to fit the image area.

Figure 4.4 is a bird's-eye view of our museum setup. Now the camera has a very-wide-angle lens with about a 90-degree horizontal angle of view.

Look what has happened to our family of angles. The family of angles causing direct reflection has grown much larger,

4.3 Placing the light inside the family of angles caused an unacceptable hot spot and obscured some of the detail.

4.4 The family of angles has grown much larger in this arrangement using a wide-angle lens. The result is a small range of acceptable lighting angles. Only the light outside the family of angles will produce glare-free lighting.

family of Ang/es family of Ang/es and the range of acceptable angles for copy lighting is much smaller. The light now needs to be much farther to the side to avoid unacceptable direct reflections.

Shooting a copy with the camera in this position would yield drastically inferior results if we kept the light where we had it in Figure 4.1. The same lighting angle that works well when the camera is farther away can cause direct reflection if the camera is closer. In this case, we would have to move the light farther to the side.

Finally, notice that in some museum-like situations, the shape of the room may make the placement of the lights more difficult than that of the camera. If it seems impossible to position the lights to avoid direct reflection, we sometimes can solve the problem just by moving the camera farther away from the subject (and using a correspondingly longer lens to obtain a large enough image size).

In Figure 4.5, the room is too narrow to allow easy light placement, but it is deep enough to allow the camera to be placed at almost any distance. We see that when the camera is farther from the subject, the family of angles that produces direct reflection is small. Now it is easy to find a lighting angle that avoids direct reflection.

Easy Model Reflection Light
4.5 A copy setup using a long lens. Because the family of angles that produces a direct reflection is small, finding a good place to put the light is easy.

The Success and Failure of the General Rule

Texts that attempt simply to demonstrate basic copy work (as opposed to general lighting principles) often use a diagram similar to Figure 4.6 to represent a standard copy setup.

Notice that the light is at a 45-degree angle to the original. There is nothing magic about such an angle. It is a general rule that usually works—but not always. As we saw in the previous example, a usable lighting angle depends on the distance between the camera and the subject and the resulting choice of lens focal length.

More important, we need to notice that this rule may fail to produce good lighting if we do not give attention to the distance between the light and the subject. To see why, we will combine the principle in Figure 4.1 with that of Figure 4.6.

In Figure 4.7, we see two possible light positions. Both lights are at a 45-degree angle to the subject, but only one of them will produce acceptable lighting. The light that is closer to the subject is within the family of angles that produces direct reflection and will cause a hot spot on the surface. The other light is far enough away to be outside the family of angles and will illuminate the surface nicely.

4.6 The "standard" copy setup sometimes produces good results and sometimes does not. A usable lighting angle depends also on the distance between the camera and subject and the choice of lens focal length.

4.7 The importance of the distance from the light to the subject. Both of the lights shown are at 45 degrees to the center of the subject, but only one is satisfactory. The light inside the family of angles will produce direct reflection.

So we see that the 45-degree rule will work fine if the photographer gets the lights far enough away from the subject surface. In fact, the rule often does serve well because photographers generally do move the lights farther away from the subject for yet another reason, to obtain even illumination.

The Distance of Light

Up to now we've only considered the angle of the light, not its distance. But clearly that's important too, because we know that diffuse reflections get brighter as the light gets closer to the reflecting surface. Figure 4.8 revisits an earlier arrangement, now emphasizing the distance of the light.

Once again, we are using a wide-angle lens to photograph the subject. Remembering that such situations leave a very small range of angles of illumination that do not cause direct reflection, we have positioned the light at a very shallow angle to the surface. But the edge of the subject that is closer to the light receives so much more light than the edge farther away that uniform exposure is impossible.

Figure 4.9 shows the resulting exposure. The shallow lighting angle avoids direct reflection, but the diffuse reflection on one side of the image is so bright that the consequences are almost as bad.

Display Case

4.8 The shallow angle that avoids direct reflection is also more likely to cause uneven illumination if we don't take care to avoid it.

4.9 A possible consequence of the situation shown in Figure 4.8. Although the light placement avoided direct reflection, the illumination is too uneven to preserve detail on both the left and right sides.

4.9 A possible consequence of the situation shown in Figure 4.8. Although the light placement avoided direct reflection, the illumination is too uneven to preserve detail on both the left and right sides.

Obviously, a second light on the other side of the subject would help provide more even illumination. (This is exactly why most copy setups do, indeed, use two lights.) With extremely shallow lighting angles, however, the second light still does not provide uniform exposure. We simply get two overexposed areas instead of one, with a dark area in the center.

One solution to this problem is to move the light closer to the camera. (An extreme example of this is a flash mounted directly on the camera.) Then the light is roughly the same distance from all points on the surface, and the illumination is more even. But this solution is also likely to place the light in the family of angles that cause direct reflection, which is a worse problem.

The only solution to this problem that always works is to move the light farther away from the subject. In theory, a light that is an infinite distance away will produce exactly equally bright diffuse reflections at all points on the surface, even at the most shallow angle. Unfortunately, a light an infinite distance away is also likely to be infinitely dim. (We will not even begin to deal with the problems of finding a light stand that high.)

In practice, we do not usually need to get the light quite that far away to obtain satisfactory results. We just need to get the light far enough from the subject to produce acceptably even illumination, but we need to keep it close enough for acceptably short exposure times.

We could offer you mathematical formulas to calculate an acceptable distance between the light and the subject at any given angle (and for any given acceptable side-to-side exposure error), but you would not use the formulas because you do not need them. The human eye is good at judging the acceptable compromise distance, provided the photographer is aware of the potential problem from the start. Place the lights so that the illumination looks reasonably even; then double-check that judgment by measuring various points on the surface with a light meter.

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