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This image shows a wide range of tonal values from the dark area of trees to the white walls of the lighthouse. The clouds in the sky offer a good middle tone and this is where I metered the scene.

We've come a long way in this column. Understanding the purpose of aperture and shutter in determining exposure and knowing how to meter a scene and where to place tonal values elevates your photographic images significantly.

Last time I introduced the Zone System as a methodology for placing tonal values.

Let's review with an example. The lighthouse image shown here was taken on Campobello Island in Canada, off the coast of Maine. I highly recommend it. This photo was taken at sunset and the light, coming from behind me, was hitting the building wonderfully. There is a wide range of tonal values in this scene from the buildings white walls to the dense, dark, evergreen trees. How do you meter such a scene? The easiest object, for me at the time, was to meter the gray middle tonal value clouds just above the lighthouse. This is what I wanted to place as a middle tone, or Zone V. Let's assume the exposure was f/5.6 at 1/250 of a second.

On what zone will the white building fall? How about the dark evergreen trees? Assume that metering the white wall yields an exposure of f/5.6 at 1/1000s of a second. That means the wall has 4 times the reflectance of the gray clouds. or 2 stops more reflectance. If I were to take the picture at that exposure, the white walls would render as a middle tone gray. But, now that I am a smarty-pants photographer form reading this column, I know that I want the white wall to appear white. I n order for that to happen I still want to expose at f/5.6 at 1/250s so that the white wall gets 2 stops more exposure and , thus, will fall on Zone VII.

Reading the table included with this article, we are able to visually place tonal values and see how they relate to other values in the scene. F/5.6 at 1/250s will render as middle tone gray, the evergreens will receive 2 stops less exposure and thus fall on Zone III and the white wall receives 2 stops more exposure and falls on Zone VII.

It should be apparent that if we were to expose the image at f/5.6 at 1/1000s, rendering the white wall as a middle tonal value then the gray clouds would end up on Zone III and render darker and the evergreens would fall on Zone I and have no detail.

There is a bit of math needed to properly explain the relationship between aperture and zones. I am assuming that there is a reader out there, somewhere, that wants to know this. If so, read on. Reviewing the Zone System table from the last column, you might wonder how the percent reflectance is determined for adjacent zones. For example, Zone V objects have an 18 percent reflectance and Zone VI objects reflect 25 percent of the light. Remember, there is a 1-stop difference in light between these zones, that is, Zone VI is 1-stop more light than Zone V. That means if a middle tone valuer (Zone V) meters at f/8 at 1/500s, then a Zone VI value would meter at F/11 at 1/500s. Right? With me so far?

Mathematically, everything works because the relationship between the reflectance values and the area change between aperture f/8 and f/11 is the same. That relationship is simply a multiple of the sqrt (2). Recalling that aperture creates a diaphragm opening through which light passes, the opening at f/8 is larger than the opening at f/11. Since the area of the opening must be double, in order to let in twice as much light, the value 11 is derived as sqrt(2) times 8. Similarly, since the relative opening must also allow for the same reflectance percentage as used by the Zone System, 25 percent is equal to 18 percent times sqrt(2). Ta Da! It all makes sense, doesn't it? Hello? Anyone still reading?

Well, now you know it all. You understand that exposure is achieved with a combination of aperture and shutter, that cameras meter to middle gray and that by adjusting one or both of aperture and shutter you can take control of placing tonal values where you want them. And, to our great relief, there is a sound mathematical reason as to why it all works.

Felling better? Ready to tackle the world and come back with awesome exposures? I hope so. Oh, as a warning, there is a test next time. After that, I will concentrate on a number of photographs demonstrating the anatomy, so to speak, of a shoot . Then, we can move on to a discussion of composition. Until then, burn the midnight oil, eat coffee grounds straight out of the can with no water added, and keep shooting.