Exposure refers to the length of time the film (or in the case of digital photography, the sensor) is exposed to light. This is controlled for the most part by two factors: the size of the aperture and the shutter speed. You can create your picture by capturing as much light as possible in the shortest amount of time or by capturing a small amount of light over a longer period. The difference is what gives you creative control over your image.

For example, a very fast shutter speed will freeze action: water droplets will be suspended in mid-air, a speeding car will be frozen in place. A slower shutter speed will cause a waterfall or that speeding car to appear blurred, giving the image a sense of motion. For stationary subjects, the aperature will allow you to control how much of the image is in focus, blurring the background and thereby giving the central object more emphasis or giving a distant mountain sharp clarity.

The last factor in exposure is ISO sensitivity. If you're dealing with film, the film you buy will be marked with its ISO rate on the box: 100 speed film or 400 speed film, etc. and you have only to tell your camera which type of film you're using. If you're working in the digital world and your camera allows this setting to be changed, you decide what ISO rating you want to simulate and set the camera for it. Film with a high ISO sensitivity is susceptible to image "noise" -- so the higher the ISO number, the grainer the images are likely to be. However, for low light or night time conditions, ISO 400 or higher films may be your best option, especially if you want to avoid using a flash.

Remember, though, that shutter speeds, aperture settings, and ISO sensitivity numbers generally indicate a halving or doubling of the exposure. So, 1/30th of a second (1/30s) will let in twice as much light as 1/60s and half as much as 1/15s. F/4 lets in twice as much light as f/5.6 but only half as much as f/2.8. And using the same aperture and shutter speed settings ISO 200 film will react twice as fast as ISO 100 film, but only half as fast as ISO 400 film.