photography 201: distance and light

Last photo 201 lesson we discussed how light "sees" your subject, and how important it is to consider the lighting angle when composing your photo. This week we'll discuss (courtesy of Strobist) the relationship between lighting and distance. FYI - this is a more advanced lesson, in that it discusses off-camera lighting, such as an external flash or a strobe light of some sort.

Strobist explains this relationship using the Inverse Square Law - thankfully he does not actually provide any equations or proofs in this discussion (because I would have dozed off right then and there). Here is how he explains Inverse Square Law with respect to lighting:

• "The closer you are to the light source, the more powerful the light. Get real close and it gets really powerful. Get far away, and it gets weaker."

So far, this seems logical right? For example if you use your on-camera flash and you are 2 feet from your subject, the light will be MUCH brighter than if you were 10 feet away, or 20 feet.

On to the next principle:

• "The closer you get to the light source, the quicker the lighting values change as you move in. When you get farther away, small differences in distance (from the light) become meaningless."

OK, this statement made me reach for my 3rd cup of coffee. Thankfully there are several demos on flickr that help to illustrate these points, such as this one:


Looking at this photo diagram, you can see: (1) the photo on the left (light source 1 foot from The Bride, or Uma Thurman) produces a more intense light on the subject than the photo on the right (light source 5 feet from Uma). NOTE - both the camera and Uma NEVER move in this exercise - only the light source is changing. As the light source moves further from Uma, the flash output becomes stronger. Also in each photo the light source is moved further back from Uma.

You can see from this photo diagram that by moving in close to the subject (photo on the left), the wall behind Uma Thurman is almost black. The flash illuminates Uma, and very little of what's behind her. As the light source moves further back, though, the difference in the shade of the gray wall is minimal, and the photo is more evenly lit throughout.

So let's review the above statement:

"The closer you get to the light source, the quicker the lighting values change as you move in. When you get farther away, small differences in distance (from the light) become meaningless."

To paraphrase, the closer the light is to the subject, the more dramatic the lighting, however, as the light is placed further and further from the subject, the differences become less and less dramatic.

Strobist describes this concept as "lighting depth of field":

• Light Placed Closer = more powerful, and control of the depth of the correct exposure.
• Light Placed Further = less powerful, and a broader zone of even lighting exposure.
  

In other words, light placed closer to your subject is like working with shallow depth of field. Light placed further from your subject is less powerful, but provides a more evenly-lit photograph.

This is not an easy concept to grasp, but it's a pretty powerful concept once you understand it. Click here for the full lesson at Strobist - there are homework assignments to help you understand the lessons more fully. I'd like to do the "homework" for this lesson - I'll post the results as soon as I complete the homework (no promises on when that might be...)