Stop the Madness
So…what is a stop exactly? Simply put, if you take a given amount of light and double it, you are moving up one stop, if you cut the amount of light in half then you have gone down a stop. So a stop isn’t a specific amount of light, instead it is a measure of difference between the amount of light. To be completely technical about it, the term “one f-stop” refers to a factor of √2 (approx. 1.41) change in f-number, which in turn corresponds to a factor of 2 change in light intensity. So how can we visualize this easily? If we have a single light bulb as a base light source, then two light bulbs would increase the light by 1 stop. Going to 4 light bulbs doubles the output again giving us 2 stops, to get to 3 stops we then need 8 light bulbs, etc.

So let’s take the sentence in the introduction, if we wanted to expose one side of the face 1 stop under the other side of the face, what we are saying is we want half the amount of light on the shadow side that we have on the lit side. Pretty simple concept huh?

For the most part, we are actually going to be doing a lot of guesswork unless we have good light meters and can measure each piece of the scene. Using our DSLRs we can zoom into each area of the scene and look at the exposure meter in the camera for a pretty good meter reading as well.

How aperture and shutter affect light
Ok, so now we have a basic understand of what a stop is, now we need to see how to control this using the two basic controls, shutter speed and aperture. To illustrate the effect of aperture, we have created this chart that shows the amount of light that is gathered at typical aperture settings. While each camera/lens may have slightly different available aperture numbers, they are typically in multiples of 1.4 (as described in the previous section). As you increase the f-stop number to the next 1.4 increment, you halve the amount of light.

The effect of shutter speed is the same concept. Every time you double the shutter speed, you decrease the amount of light in half. If you cut the shutter speed in half, you double the amount of light.

Again, you can see this represented by a similar chart.

Where this gets interesting in combining aperture and shutter speed, you can get the same exposure with different combinations of aperture and shutter speed. For example, a shot taken at 1/500 at f/2.8 and a shot taken at 1/60 at f/8 will have the same equivalent amount of brightness, what will change is the depth of field.

Let’s take a look at one of our favorite studio lights, the Westscott TD5 Spidelite. The light head has 5 bulds in it, you can control the center bulb, and two sets of two bulbs. To show how this affects stops we can use the different combinations to control how many stops of light we have:

If we only turn on the center bulb and use that as our base reference point, we can then use the different variations to come up with the following amounts of light changes:

How can we use this information?

If you have a light meter than putting this information to use is just a matter of measure-adjust-repeat until you get the desired light readings you want. With a modern DSLR, we can do this with the camera’s light meter with fairly good results.

Let’s say we want to use a two-light setup to light a subject’s face and we want one side 1 stop darker than the lit side. To begin with we zoom into the lit side of the face (or use the Photovision Digital Calibration target aimed at the light source , the small size works best for this) and adjust the lights until you get a good exposure reading. This is the time to make the camera adjustments to center the exposure in your camera’s meter.

Good Exposure Reading

The next step is to meter the shadow side of the face, using the same technique of zooming into the subject or a target to get a meter reading. This time however we want to underexpose by one stop.

-1 Stop Under Exposed

At this point we now have the camera setup for the right exposure on the lit side of the face and we checked to make sure the shadow side was one stop underexposed, we are now good to take our shots and be confident that the lighting will turn out good.

Summary
Hopefully this has helped to explain the concept of stops as it pertains to the relative amount of light and how we can use the tools we have available to make sure we have good lighting and exposures. To discuss this with others, please join us in our Camera Dojo Flickr Users Group.