Quality of Light - part 2 (Hard Light)

Continuing our discussion of the quality or contrast for light, we have seen that a small light source produces high contrast light that makes well defined, hard shadows this type of light is often referred to as hard light. Conversely, we saw that a large light source produces low contrast light that makes soft shadows. This type of light is often referred to as soft light.

The problem with this discussion so far is that we haven't really defined what constitutes a large or small light source. We have so far also neglected the effect of distance between the light source and the subject (coming in part 3). For now I will just say that the size of the light source is always relative to the subject. Assuming the light source is relatively close to the subject, then a my idea large is when the size of the light source approaches or exceeds the size of the subject. Small would be a light source that is less than somewhere around ¼ the size of the subject.
Now that we have established some ball park metrics for size (grabbed out of thin air for the sake of discussion), let's take a look at why size matters. The following diagrams attempt to explain this, but might need a little help, so I'll do my best to describe what they are trying to show you (without getting too verbose).

For the following diagram, assume that light projects evenly from across the entire front of the light source. Also assume that the light source has a perfect reflector that focus the light perfectly at the angle shown by the yellow area (we only care about the light in the middle anyway).

In this first diagram we have a (near) single point light source. Obviously it is tiny compared to our subject. I've use colored lines (vectors) to show the rays of light emanating from this source where they just touches the sides of our model's face. Past this point behind the model, on one side of the line we have light that hits the background. On the other side the light is blocked by our model and so a shadow gets created. This shadow is clearly defined, as there is no light reflected or otherwise directed into the shadow area. What we have here is some very hard light.
In the next installment I will show how a larger light source creates a softer shadow.

Continue on to "Quality of Light - part 3 (Soft Light)