Three important settings affect how sharp your photos will turn out: shutter speed, aperture and ISO. Because each of these settings affects the others, understanding how they work and what they do is vital to getting the sharpest photos in different circumstances. Of course, these are not the only components in getting the best photos. Properly focusing your camera, ensuring your lenses are clean and saving your images in the largest format possible will also affect how sharp you photos appear.
1Shutter Speed: The longer your shutter is open, the more likely your photos will be blurred by movement -- either that of your subjects or of your own hands. To eliminate hand tremble, a tripod is an essential accessory for serious photographers. The more you zoom in, the more hand tremble becomes a problem. A good rule of thumb is to use a shutter speed of 1/60th of a second or faster if you are using a 50mm lens. For a 100mm lens, use a shutter speed of 1/125 or faster. For a 200mm focal length, shoot at 1/250 or faster. Keep in mind that a faster shutter speed means you will need to make your aperture wider -- triggering a fresh set of concerns.
2Aperture: The aperture controls how much light is let into the camera. The wider your aperture is, the more light comes in. However, widening the aperture also reduces the depth of field, and reduces the quality of fine details. Using a lower number like f/4 makes it harder to focus and objects in the background and foreground will be blurred. A smaller aperture, indicated by a higher setting such as f/20, makes it easier to focus, and objects in the background and foreground are also sharper. However, because the smaller opening lets in less light, you must decrease shutter speed for proper exposure. It's important to balance aperture and shutter speed to achieve the sharpest photo.
3ISO: In film photography, ISO determines how sensitive the film is to light. With digital cameras, it determines the sensitivity of the camera's light sensor. The higher the ISO, the more noise and less sharpness you will get in your photos. In low-light settings, or for a fast shutter speed when taking an action shot, a higher ISO of 400 or even 800 might be required. In a well-lit environment, you can usually keep the ISO as low as 100; however, the shutter speed may need to be slowed as a consequence.
4Understanding Your Camera: Taking lots of photos and comparing the results of different settings is the best way to know what you can expect from your camera, and what settings you should use in different situations. If you are using a telephoto lens, use a tripod. If you are using automatic focus, take a moment to look at what you see in the viewfinder before taking the picture. If autofocus doesn't give you the focus you need, try turning it off. When you can control the setting, like taking a group photo at a family event, take some time to ensure you have good lighting so you can use a low ISO, and then take a few pictures with different aperture settings.