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Light Behavior Basics
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PostPosted: Mon Mar 12, 2007 2:17 pm 
Light Behavior Basics

There are fundamental properties of lighting that form the basis of our value judgments concerning its effectiveness. Many of these properties are not understood fully by the novice photographer, and even some professionals.


Contrast

Contrast is the degree of separation between light tones and dark tones. In the basic context of defining contrast, we might think of a picture of a black and white striped surface as being “high contrast.” While this is the correct line of thinking, there is a far more complex relationship to lighting and subject matter and contrast than initially comes to mind.

Consider as a model, for instance – a brown skinned person in a brown suit with a brown dog against a brown background. The amateur might think the contrast of this scene is established by the subject coloration, while the professional realizes the same scene may be shot with almost any degree of contrast desired, by manipulation of the lighting equipment. Besides the obvious placement of lights to form actual gradients, or variations of light intensity across selected parts of the scene, the surface textures of the various scene components can play an extremely important part in how the scene looks under certain lighting conditions.

In order that we fully understand lighting and texture and contrast, let us conduct a mental experiment:

We shall use two light sources, each producing, say, 2000 Lumens. Light Source #1 is essentially a point source of light, measuring 1/4-inch in diameter. When we look at Light #1, it appears extremely bright and uncomfortable to look at. It appears bright since all 2000 Lumens are emanating from a very small surface area, having a very high “spot intensity.”

For Light Source #2, we will place a 2000 Lumen lamp inside a huge sphere of diffusing material (white cloth, frosted glass, etc.), thus producing a very large light source, perhaps 10-feet in diameter. When we look at Light #2, even though it too produces 2000 Lumens, it does not appear nearly as bright, nor is it uncomfortable to look at. Its spot intensity is extremely low with respect to Light #1, though its overall intensity is the same. It is a large “soft” light source.


Specular Reflections

Now, imagine a room with one wall covered in fine white felt, a black wall opposite and a light in the middle. If we look at the white felt wall, it looks the same regardless of which of the above light sources is used. In either case, it is illuminated by 2000 Lumens and there are no surface reflections to give us a clue to the size or shape of the light source.

If we replace the white felt with a large mirror, we see something entirely different. If we turn on Light #1, we see the same image as we would by looking directly at the light – a very intense spot of light. If we photographed this, we would have an extreme contrast image – a bright, overexposed spot, surrounded by a dark background. On the other hand, if we viewed or photographed the mirrored wall using Light #2, we would see a much lower contrast with a large soft glow of much lower spot intensity – a reflection of the large, soft light source.

From this experiment, we can see that the surface reflectivity has a great deal of bearing on things. While both walls are colorless, the flat textured felt appears the same under either “point-source” or “large-source” light, while the mirrored (specular) surface looks entirely different under the two lighting conditions.

If we replace the two walls with more common subjects such as the “all brown” scene described above, we will find that each surface, though brown, has its own degree of specularity, or surface reflectivity. The man’s face may have small beads of sweat and oil for instance, which form tiny mirrors. The brown shoes may be highly polished and the laces matte. The suit may have some shiny polyester threads. Given a certain level of scene illumination, the picture will appear entirely different depending upon the spot intensity and source size of the light sources – the smaller the light source (higher spot intensity) the more intense the reflections from the ”specular,” or reflective bits of the scene.

Where the specular areas are relatively large and flat, such as the polished shoes, we will actually see a discernable reflection of the light source. Where the specularity resides in the much smaller textures, such as in the polyester threads, we will simply perceive glistening highlights. In any event, the highest possible scene contrast will appear as the lights approach point-source, while lower contrast and less surface definition will prevail under conditions of high light-diffusion.

The degree of contrast we wish to convey via lighting is governed by the subject matter and artistic goals. Too-high contrast can result in pictures that are impossible to print, perhaps with glare and washed out areas or emphasized undesirable features such as wrinkles. Too-low contrast, caused by excessive diffusion, can lead to dull, uninteresting and undefined pictures. The answer is to first understand the principles involved, and learn the range of contrast that will print well and look good, and to know what it is you’re after. You can then use various combinations of diffused and direct light to achieve your goals.


Shadow Structure and Contrast

The degree of light diffusion affects scene contrast in another way, via the introduction of shadows. Again using hypothetical light sources - Light #1 and Light #2, as well as the white felt wall, let’s conduct another experiment.

This time, let’s place a new object, say a person, in front of the wall. Using Light #1, all light that falls on the scene comes from a single point. Wherever there is an obstruction, a distinct black shadow is formed. The person’s shadow appears on the wall, and dark shadows form under the chin, behind the nose, etc. The contrast is very high when the brightness of the highlights is compared to the blackness of the shadows. Wrinkles become pronounced due to the dark shadows that form with each “hill and valley” of the skin.

If we now substitute Light #2, the shadows soften or nearly disappear because the light is coming from many angles and “wraps around” any surface obstructions. Since there are no longer any deep, dark shadows and the reflections from specular surfaces are subdued, the ratio of highlight intensity to shadow intensity is much lower – lower contrast and less defined features result. Again, overly small light sources produce excess contrast (a hard look), while an excessively large light-source produces a soft, featureless result.


Two Factors Affect Contrast

Thus, while there are two distinctly different mechanisms working to associate contrast with size of the light source(s), they both work in the same general direction. To illustrate that there are, in fact, two separate parameters. To show that they can be controlled independently, let us set up a third hypothetical lighting situation:

This time, assume there are 10 point-source lights, each producing 200 Lumens, arranged in a broad array about 3-feet square in front of the subject. From the standpoint of shadow formation, there will be essentially as much “wrap around” effect as would be obtained with a 3-foot diffusion panel. Since the light will come at the subject from a broad range of angles, there will be no deep black shadows, rather a multiplicity of distinct shadows that blend to form a soft overall shadow structure.

As far as specularity is concerned, while each 2000 Lumen light has only 1/10 the spot intensity of a single 2000 Lumen light, the spot intensities are still hundreds of times greater than would be evident with a 3-foot diffuser. Each light is quite capable of producing strong reflections from shiny components of the subject. What’s more, when we are dealing with surfaces containing a large number of tiny reflective particles (perspiration, sequins, metallic threads or generally reflective surfaces of angular form such as lips, eyes, small objects), the multiplicity of small “pin” lights will produce ten times as many specular “glints” than would a single, point-source light.

Thus, while the reflections are not as intense, there will be more of them. The net result of this lighting scheme should be a picture that has a high apparent contrast and specularity, yet one that reveals no hard shadows and is much easier to print than a picture made with a single point-source light. Notice, the parameter that causes the relatively high contrast has been isolated to the reflective component, while the shadow structure appears almost the same as it would with a large diffusion panel.


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