Exposure = Intensity x Time
Intensity refers to your aperture or F-stop. The smaller the number, the larger the aperture and the more light that comes in. The “speed” of your lens refers to the largest aperture of the lens. The larger your aperture setting, the smaller your depth of field.
Time refers to your shutter speed. They are indicated in seconds or fractions of a second. A good rule of thumb for choosing your shutter speed for a shot without using a tripod is based upon the length of your lens. For whatever length of your lens (in mm), use no slower shutter speed without a tripod. For example, with a 200 mm lens, you’d want to use a shutter speed no slower than 1/200th of a second to eliminate blurriness caused by hand-shake (unless, of course you’re using a tripod). Of course this is just a rule of thumb, and if you can make it work, go for it.
Referring back to the above equation, you can see that proper exposure is a ratio of your shutter speed and F-stop. You manipulate these to get proper exposure and the desired effect. For example, a fast-moving object shot at a slower shutter speed will produce a blurred effect—not necessarily a bad thing, depending on what you’re going for. Higher F-stops produce higher depth of field, so if you had two or more object at different distances from the camera, a higher F-stop would be more likely to get them both into focus.
The length of your lenses is one way to alter the look of your photos. I read once that a great way to appreciate all that each of your lenses has to offer is to shoot for a certain amount of time with only one lens. Most people starting out in photography want to slap on their longest lens, but I found that shooting with short focal length lenses helped me to take better photos overall. For one thing, it forced me closer to my subjects—something that many people are reluctant to do, but which is often quite necessary. Don’t be afraid to fill your frame with your subject.
Research different composition techniques. Rule of thirds, leading lines, framing, balance—all great ideas to get you thinking and shooting like a photographer.
Finally, remember to move. Shoot every subject from different angles, and you’re likely to find great photos in the most mundane places. Remember that we see the world from eye level with the equivalent of a normal lens (50 mm or so with a 35mm equivalent camera), so shooting your photos from here is booooring! Get up high, get down low. Long lenses, short lenses. Make your photos show the world in ways that no one has ever seen it. Look at other people’s photos (flickr is a great site for this) and critique their photos. Find photos you like, and then figure out why you like them. Find photos you don’t like and then figure out why you don’t. Carry your camera everywhere, and look at the world as if you’re looking for your next photo.