Saturday, April 25, 2009

Quality of Light - part 4 (effective size)

So far in this section we have seen how size affects the quality (contrast) of light. A small light source produces hard, high contrast light, and how a large light source produces soft, low contrast light. In this last discussion on the quality of light I would like to show how distance between light source and subject change the effective size of a light source.

In the diagram below, both lights are the same size, but the bottom light is positioned at about three times the distance. Again I have placed vector lines at three points along the light source to represent rays of light at the points that just touches the model's face. What becomes obvious when looking at these lines is that the angle of the lines is greatly affected by distance. When the light is close, the angle is greater than when the light is farther away. In this case the close light has about 55 degrees of spread from the light rays emanating from the extreme edges of the light source. The light that is farther away has about 12 degrees of spread from those same two edges of the light source.
As you can see, because of the smaller angle, the resultant shadow is much less feathered and is the same as would be produced by a smaller light source at a closer distance. So it can be said that moving a light source farther from the subject makes it effectively a smaller light source, at least in terms of the effect on light quality.

This concludes the discussion for the quality of light. Next post we will delve into discourse on light temperature, which will be the last section for this series.


  1. Good job! One of the best explanations I have seen. Simple and short enough to keep it interesting, comprehensive enough to make it worth reading!

  2. gene, you just blew my mind. im a visual learner and your diagram answered about 4 questions at once for me. THANKS!


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