As the photographer you are often able to control the quantity of light reaching the sensor by altering one or several of the four factors that affect the exposure. If each of the four factors only affected exposure, life would be simple; but changing the light, ISO, shutter speed, and aperture all have other consequences.
Light:
Looking to see how much light is in your photograph is the starting point in assesing your exposure settings. Light is the most significant part of evaluating the scene you want to capture. For example, is the focus of your photograph in the shadows or in bright light? What is the quality of the light, and where does the light fall? Is the light a soft glow from a candle or harsh, direct light from the sun? Is part of your artistic vision the actual color of the light? The color of the light at a sunset is unlike the color of the light inside a concert hall. Despite the fact that the light in your scene is the most important part of the exposure equation, it could also be the hardest of the four things that affect exposure to change. Without extra equipment, decreasing or increasing ambient light is a difficult task. It may be possible to physically move yourself or your subjects, but this changes the composition of your photograph. Fortunately, many products are available to help shape, modify, increase, or decrease the light in your scene.
Aperture:
Aperture is measured in f-stops and is written as f/2.8, f/3.5, f/4, f/5.6, f/8, f/11, and so on. The higher the f/number (f/22), the smaller the opening—allowing less light to reach the sensor. The lower the number (f/2.8), the larger the opening—allowing more light to reach the sensor. Basically, an f-stop is a unit of change. For instance, when changing by whole stops, f/5.6 lets in half as much light as f/4, and f/2.8 lets in twice as much light as f/4. Even though changing the f-stop results in more or less light getting through the lens, this action has other consequences as well, as explained below:
Depth of field:
When you use aperture to modify the exposure, you are also changing the depth of field. The depth of field (DOF) is the remoteness in front of and behind the subject that appears to be in focus. As only one plane of an image is in exact focus, everything else is technically out of focus. The part in front of and behind this focus point that is still acceptably sharp is called the depth of field area. The depth of field area starts one-third in front of the focus point and extends two-thirds in the rear. When you use a smaller f-number (larger aperture opening), you get a shallower depth of field. When you use a larger f-number (smaller aperture opening), you get a deeper depth of field. This depth of field control lets you selectively focus on various items in the same view. Landscape photographs and big group portraits have to have a greater depth of field; both the foreground and the background must be in focus.
The other thing that affects depth of field is the distance of the camera from the subject. The bigger the distance between the camera and the subject, the bigger the depth of field area. To increase the depth of field, use a smaller f-number, move farther away from the subject, and make use of a shorter focal length. To decrease the depth of field, use a larger f-number, move closer to your subject, and use a longer focal length.
Shutter speed:
Shutter speed is measured in fractions of a full second (unless it is over 1 second). It controls the length of time that the shutter is open and letting light reach the sensor. As changing the shutter speed affects the quantity of light reaching the sensor, it has a direct effect on exposure. Increasing the shutter speed reduces the amount of time that the shutter is open, letting in less light. If you decrease the shutter speed you will increase the amount of time the shutter is open, letting in more light. When you increase the shutter speed by one full stop, you halve the amount of light that reaches the sensor. When you decrease the shutter speed by one full stop, you double the amount of light that reaches the sensor.
Changing the shutter speed changes the way movement is captured in the photograph. When the subject of your photo is moving, you have to decide if you want to freeze the action or show the motion. A faster shutter speed, such as 1/2000, freezes the action; and a slower shutter speed, such as 1/15, produces motion blur.
- Motion blur. Sometimes you may prefer to show motion in your photographs. By leaving the shutter open longer, objects in motion in your scene do not appear in sharp focus; they instead look blurred, and the nonmoving items in the scene are in sharp focus. You can also use a longer shutter speed and follow the action with the camera. By keeping the camera moving at the same speed as the subject while the shutter is open, the subject stays in focus and the background blurs. This technique is called panning.
- Freezing action. To capture a moment in time, the shutter of the camera opens and closes in a frac- tion of a second. To stop a girl running in a park the shutter needs to be open for a very small amount of time to avoid blur. The faster the action, the higher the shutter speed needs to be. The general guide for stopping motion is 1/60 second or faster. When shooting a fast-moving scene, setting the camera’s exposure mode to Shutter Speed Priority mode (S) lets you concentrate on freezing the action while the camera adjusts the f-stop.
ISO:
The higher the ISO number, the less light you need to produce a correctly exposed photograph. However, when you use a higher ISO value, you introduce digital noise. At a higher ISO setting, the camera amplifies the signal from the sensor, resulting in more background electronic noise in the photo. When too much light gets to the sensor, the images are overexposed; too little light and the images are underexposed.
A bright sunny day has plenty of light, enabling you to pick from a variety of f-stops and shutter speeds. A more challenging situation occurs when there is not a lot of light. Getting the correct exposure is a balancing act: As the amount of light decreases, you can increase the ISO, decrease the shutter speed, or use a smaller f-number. As I described the pros and cons of each, you can now decide what settings you want to change. By keeping the f-stop at the optimal f-stop, you get the sharpest focus in your photos and not too shallow a depth of field. By keeping the ISO as low as possible, you can avoid introducing noise into the photograph. By keeping the shutter speed high, you can freeze the action and avoid motion blur.
See also: Understanding Exposure, Understanding focal length.
And of course the Photography tips page with a list of articles that might help you improve your photography skills.