Learning About Exposure and Lighting
Even with all the processing power inside, there's only so much that your digital camera can do. You can ask it to take a photo in the full sun with your subject between you and the sun...or you can try taking a photo after dark with a handheld camera - but the results probably won't be as good as you'd like. Once you understand the basic concepts of exposure and lighting you'll be much better equipped to use your digital camera to its full abilities.
Getting a correctly exposed image is all about getting the right amount of light stored by your digital camera's sensor. There are four factors to consider: the amount of light allowed to come through the lens (adjusted using the "aperture" setting of your lens), the amount of time the sensor is exposed to the light (the "shutter speed"), the sensitivity setting of the camera sensor (often call the "ISO" setting) and the amount of light reflected off your image's subject. Digital SLR's (and some point-and-shoot cameras) let you control the first three factors, while adjusting your position or using a flash can help with the fourth.
Aperture
The aperture of your lens is the size of the opening that allows light to come through. The aperture is usually described in terms of an "f-stop" (written as something like F2.8 or F32). The smaller the "f-number", the wider the opening. The bigger the aperture the more light that will come through while the sensor is exposed to the light. That will mean that the other exposure factors have to be adjusted to balance out the effect of the aperture.
Why not just stick with a set aperture and adjust the other factors as needed? This is actually what happens with many lenses on point-and-shoot cameras. But aperture also plays a role in what's called the "depth of field" of an image. The depth of field describes how much of the image will appear to be in focus. For instance, a larger aperture (say, F2.8) may result in an image where only those objects that are between 2 feet and 3 feet from the camera appear in focus in the final image. Meanwhile, using a small aperture (such as F32) can bring almost the entire image in focus. The ability to adjust the aperture gives you the ability to do some creative things with depth of field. It can also act as a balance when you want to control your...
Shutter Speed
The shutter speed is the amount of time that the shutter of your camera is open and the sensor is exposed to incoming light. (In modern cameras the shutter doesn't open and close, it's actually a small slit that quickly moves across the face of your sensor.) It's represented in terms of fractions of a second (or number of seconds, for really long exposures), something like 1/60th of a second or 1/2,000th of a second. As mentioned above, a faster shutter speed can be used to balance out a wider lens aperture - to make a "correct" amount of light fall upon the camera sensor. But shutter speed can also be used to capture things that are moving very quickly (without seeing a blur in the final image) or to show motion (allowing the blur to be visible with a slower shutter speed).
Sensor Sensitivity (ISO)
So you have the ability to control the shutter speed and the aperture to try to get the "right" amount of light to the camera's sensor. But what is the "right" amount of light? That depends upon the sensitivity of the digital camera's sensor.
In the good old days of film, you had the same kind of sensitivity factor controlled by the "speed" of the film you chose to use. Film with a smaller "ISO" (formerly known as "ASA") number had less sensitivity and required more light (bigger aperture, slower shutterl speed). What was the difference between the different film speeds? Graininess. Faster films usually had visible "grain" in the resulting images. We won't go into the technical details here, but it had to do with the amount of silver in the film and how it was applied. Common film speeds were between ISO 25 and ISO 800.
Digital cameras make switching to a different "film" much easier - all digital SLR's let you change the "effective ISO" of the camera's sensor at any time. No more switching between rolls of film, just turn a dial on the camera. Interestingly, the issue of graininess still applies to the ISO setting you choose. The faster the "film" (higher ISO number), the more evident pixels will be in the final image. It has nothing to do with silver and chemicals, it's just a weird coincidence that sensor technology has similar problems when the sensitivity is adjusted.
This is getting complicated
Yes, it is. Fortunately, all digital cameras have an "auto" or "program" setting that will take care of all of it for you. The camera will do it's best to adjust everything to correctly expose the image. And in non-extreme situations, you'll usually get a good photograph. But the camera can be fooled and knowing when that can happen lets you "take charge" and avoid problems.
Don't like the results from "auto" or "program" mode? The next level of flexibility is using "aperture priority" or "shutter priority" mode. In aperture priority (AP) mode (it may be called something slightly different on your camera) you choose the aperture and the camera picks a "correct" shutter speed. Conversely, in "shutter priority" mode (SP) you choose the shutter speed and the camera uses its internal metering system to choose a matching aperture.
Still not quite right? Investigate your camera's "exposure compensation" system (again, may be called something similar). That lets you make micro-adjustments to the overall exposure (usually in 1/3 or 1/4 f-stops) while leaving the other settings intact.
Finally, you can go "manual". This is the choice of pros who have the time, tools and equipment required to determine the proper settings (with the help of an external exposure meter). Gives you complete control to create just image you're going for.
