Exposure Basics
Getting proper exposure requires the right balance of light, lens opening size (aperture), duration of exposure (shutter speed) and while technically not part of "exposure", proper sensor sensitivity (ISO setting) is required. In discussion of exposure, the term f-stop is typically used. A change in exposure of 1 f-stop allows half as much light (if the camera is stopped down - less light) or twice as much light (if the camera is opened up - more light) in the final exposure.
Before going on, I'll briefly talk about proper exposure. In normal shooting conditions, I strive to get the final image exposure correct at the time of capture. I use the histogram to monitor my results. If I see pixels stacked at the left (blocked shadows) or right (blown highlights) on the graph, I know that I have lost shadow or highlight detail (all pixels are the same color - black or white). At that point, I have to accept the detail loss or adjust my exposure (or shoot multiple tripod-stabilized frames at different exposures for HDR merging in Photoshop later). Canon's latest generation DSLRs allow great latitude in exposure adjustment during post-processing, but increasing brightness increases noise (digital grain) in the final image. I try to keep the graph on the histogram where it should be for the final image.
That said, I try to error on the high side of the histogram - slightly higher than desired exposure. This technique is often described as "shooting to the right" or "ETTR" (exposing to the right). Actually, some photographers shoot with an exposure even higher than my slightly higher one. Setting your exposure so that the brightest pixels in the image are near/at the right side of the histogram insures that the best detail is retained in the shadow areas of the image. More light from the dark areas of the image is able to reach the sensor - allowing it to capture more and better quality detail. The exposure for these images is then adjusted to perfection during post-processing.
Now, back to ... Getting the proper exposure requires the right balance of light, lens opening size (aperture), duration of exposure (shutter speed) and sensor sensitivity (ISO setting).
I'll leave the discussion of light to another day. Suffice it to say that shooting under bright sunlight is dramatically different than shooting under dim tungsten lighting.
The aperture setting controls how much light is allowed through the lens. Allow less light through, get a darker image. Aperture values are a relationship of the lens opening to the lens focal length. Each full f-stop represents an opening with twice as much area. Typical full f-stop increments are (from wide to narrow) f/1.4, f/2.0, f/2.8, f/4.0, f/5.6, f/8.0, f/11.0, f/16 ... A change in aperture of one f-stop requires a change in one of the other exposure parameters by the same amount to retain the same final image brightness. Keep in mind that aperture also controls DOF (Depth of Field). The wider the lens is opened, the less DOF there is in the image. You must determine how much of your image needs to be in sharp focus - or how blurred you want the background. Lenses are typically sharper and have less vignetting when they are stopped down 1 or 2 stops from wide open. I discuss aperture in this Canon Lens Recommendations page as well.
The shutter speed setting determines the duration of light reaching the sensor. Allow the shutter to stay open longer, get a brighter image. If the aperture is changed by 1 f-stop, the shutter speed is required to be changed by an equivalent amount to retain the same exposure. For example, if the aperture is stopped down from f/2.8 to f/4, the exposure duration must be doubled - from 1/250 seconds to 1/125 seconds for example. Slower shutter speeds/longer exposures are not able to mask/stop camera and subject motion as well as fast shutter speeds/shorter exposures. At times, motion in the picture is desired. A rule of thumb for handholding a camera is to use a shutter speed of 1/(the focal length) x FOVCF. You need to find your personal requirement for handholding, but this is a good starting point. To stop sports action often requires an exposure duration of very-minimally 1/500 second and 1/1000 or higher is often better. Doubling or halving the shutter speed requires a similar change in one of the other parameters to achieve identical exposure.
The last parameter is the sensor's sensitivity to light - the ISO sensitivity setting. The camera is able to amplify the light its sensor receives by specific, user-adjustable amounts. Full stop settings are from low sensitivity to high sensitivity - ISO 100, 200, 400, 800, 1600 and 3200 for example. I generally shoot with the lowest ISO speed that will give me the other parameters I need for the shot. Why? As the sensor is called upon to amplify the signal it is receiving (higher ISO setting) to increase image brightness in low light, lower dynamic range, increased noise and decreased detail are the results. This is an area where the latest Canon Digital SLRs perform extremely well compared to film SLRs. Also - Do not trade ISO speed for a blurry image. It is rarely worth it.
As you progress in your photography, you will want to take more control of the various settings that control exposure. Learning to use the creative modes will allow you to control the power of your Digital SLR. Your manual is a great source for learning these modes - and experience/experimentation is perhaps the best teacher. My most-used mode is M (you select all parameters) followed by Av (you select the aperture and ISO) for use in changing light conditions. I suggest you learn to use these.
Want to learn more?
The book Understanding Exposure is a very popular one.
