I also had the clever idea of weaving my own style of erotic photography into the project, starting with a play on the word 'exposure'. The first image in what was to be a series demonstrated light exposure by drawing a parallel to exposure of the model's body. I had it in my head to do other shots with similar parallels related to shutter speed, aperture size, and ISO Level, but I never got around to finishing the series. That was in the spring of 2010, and now it's nearly fall 2013. But I've revisited the idea, created all new images (and mostly new descriptions), and finally finished the series.
Here it is!
The Principles of Exposure
At its heart, photography is a process that captures and records light. So, an important question to ask yourself before taking a shot is, "am I getting the right amount of light?" Without adding or subtracting light to the actual scene (e.g., switching on a lamp, closing the blinds), there are a number of parameters that can be adjusted within your dSLR camera to modify how much light the camera picks up.
Understanding how to manipulate these parameters to control the exposure (the amount of light the camera reads) is one of the fundamental skills that separates the amateur snapshot photographer from the serious hobbyist or professional. Knowing how each of these parameters works, and the effects it has on the image your camera records, will help you to intuit which of them needs to be adjusted to get a proper exposure in any given situation.
The question of "proper" exposure is, itself, often a subjective one. But modern cameras do a pretty good job of guessing, so you should learn to read the exposure meter on yours. Photos that are underexposed (not enough light) will come out dark and shadowy, while photos that are overexposed (too much light) will be bright and blown out. Either of these effects may be desired for artistic purposes (e.g., silhouettes, or high key photography), but generally you want to aim for that happy medium.
Underexposed images will be dark; overexposed images will be bright - aim for the middle.
Shutter Speed (side effect: motion blur)
When you "click the shutter", prompting your camera to take a picture, a gate opens, allowing light from the scene your camera is pointed at to reach the sensor (or film) behind the lens, to record the image it sees. The amount of time that gate stays open is determined by the shutter speed, which you can adjust. Fast speeds (often measured in fractions of a second) yield short exposures, and slow speeds (often measured in numbers of seconds) yield long exposures.
The longer the shutter is open, the more light the camera will pick up, increasing the exposure; but if you or your subject is moving, a slow shutter speed will produce an image with motion blur. This may be desired in some instances, where you want to depict movement (such as the headlights of moving cars, or the running of water in a stream). There are also some neat tricks that can be done with long exposures, such as light writing and ghost effects.
A fast shutter speed freezes motion; a slow shutter speed introduces motion blur.
However, often a clear image with sharp details is desired, especially in cases where you want to freeze some kind of action (like moving athletes or wildlife). This requires a fast shutter speed, which inevitably reduces the exposure. If you're shooting in low light conditions, I suggest using a tripod for stability. Additionally, you may have to consider adjusting another parameter to ensure that the exposure is bright enough.
Aperture Size (side effect: depth of field)
Aperture size is the size of the opening the light passes through when you click the shutter, and is another parameter you can adjust in your camera. Larger apertures take in more light than smaller apertures, resulting in a brighter exposure. Aperture sizes can be confusing to read, because they are measured in "f-numbers", with the number being the denominator of a fraction, so that f/1.8, for example, is a larger aperture than f/8.
A large aperture produces a narrow depth of field, where only objects within a small distance from the focal plane (where the lens is focused) are rendered in focus. Any objects beyond that distance will appear blurry. This creates an aesthetic effect that is very popular in photography (see: bokeh), and it also helps to focus the viewer's eye onto a single object in the image.
A large aperture yields a shallow depth of field; small apertures go deeper.
However, if you want objects at different distances from the camera to be simultaneously in focus (like a person's face, and the finger they're pointing at you), you'll need a wider depth of field, which can be acquired with a smaller aperture. But beware, your resulting exposure will be darker - you'll have to adjust some other parameter to brighten it up. You may also notice that with small apertures, points of light show up as star bursts.
ISO Level (side effect: noise/grain)
The last of the three important parameters you can adjust to modify exposure is the ISO Level. ISO Level refers to the sensitivity of the camera sensor (or film, if you're not shooting digitally) that records the light coming through the camera during an exposure. You can increase the sensor's sensitivity in order to pick up more light, without adjusting either the shutter speed or aperture size. This is especially handy when you're shooting in dark environments, and you need a quick exposure to reduce motion blur. However, the cost of greater sensitivity is a grainier image. You should try to use the lowest ISO Level possible in any situation, but don't be afraid to bump it up when the situation warrants it.
Higher sensitivity picks up more light, but produces grainier images.
If you're desperate, and there's just not enough light getting to your camera, no matter how you adjust the shutter speed, aperture size, and ISO Level, you always have the option of using your camera's flash. I use this as a last resort, however, because not only does it change the look of the scene (a candlelit dinner, for example, will lose its ambiance when lit with a bright flash), and look more amateurish (since amateurs who don't know how to adjust exposure more often resort to the flash in low-light conditions), the light produced by a direct, head-on flash is harsh and often unflattering. Use it sparingly.
Ambient light is more dynamic; direct flash is harsh and unflattering.
Of course, at this point, you may want to consider experimenting with alternative sources of light (like off-camera flash), or looking for a faster lens (which can be expensive). But then, we're talking about other areas of photography, and that discussion is for another day.