Understanding Aperture and Shutter Speed
When taking a picture there are 3 items that affect whether you can expose a picture under any given light conditions: film speed, shutter speed, and aperture (or f-stop). When shooting film the part of the equation that you typically could not change quickly was the film speed itself. Therefore the emphasis on exposure control was in the two areas of shutter speed and f-stop, or aperture. With digital photography we can easily adjust the film speed, but film speed has more to do with other characteristics of the final product than just the exposure. So there is still an emphasis on shutter speed and f-stop settings.
Let's look at how these two factors play against each other to create proper exposure.
Shutter speed is the amount of time that the shutter is open allowing light to expose the film or digital sensor. Shutter speed is also known by the terms "Exposure" and "Exposure Time."
Shutter speed is measured in fractions of a second. It can be written many different ways, but the best way is with an actual fraction. For example a shutter speed of 500th of a second is written "1/500 s." A picture can be exposed for longer than 1 second. For example, 2 seconds is written "2 s."
Exposure times have been standardized for direct correlation to aperture openings. Each shutter speed option on your camera's dial doubles or halves the speed before or after it. The same is true with aperture. This allows you to adjust each one together to get the exact same exposure on the digital sensor but resulting in different effects.
The faster the shutter speed, the larger the denominator in the fraction will be. For example 1/1000 s is twice as fast as 1/500 s. The faster your shutter speed the more you will be able to freeze the motion in an action shot and keep it from blurring. However, you need more light to accomplish this.
The aperture, or f-stop, is the iris in the camera lens that allows, or restricts, the amount of light passing through it to the film or digital sensor. This is also measured in fractions and is a very complex process. I am going to explain it on a basic, or practical, level. Therefore this is not necessarily technically accurate, but it helps you understand how it all works.
An aperture of f/1.4 is a larger opening than f/16. If you think of the lens opening as a whole unit (the f in f-stop) and divide it by the aperture number you get a fraction. The whole lens "stopped down" by a factor of 1.4 (1/1.4 or f/1.4) is still most of the lens being open. The whole lens "stopped down" by a factor of 16 (1/16 or f/16) means that most of the lens is closed.
The more open a lens is (meaning the smaller the f-stop number) the more light will get through the lens at one time. The more closed the lens is (meaning the larger the f-stop number) the less light. Also, the more open the aperture is, the smaller amount of the picture will be in focus from front to back. This is called the "depth of field." Conversely, the more closed the aperture is, the greater the depth of field, or more of the picture will be in focus from front to back.
|The picture on the left has a smaller aperture opening (f/29) while the picture at the right has a larger opening (f/14). Because of the smaller aperture, the one on the left needed a longer exposure. It was exposed at 1/60 s. The other at 1/250 s.|
Shutter and Aperture Working Together
Because each shutter speed step and f-stop number are roughly double or half of the one next to it, you can adjust each one together to get a proper exposure with various characteristics.
Here is a practical example. If you are taking a picture of a flower in normal sunlight the camera may indicate that proper exposure is 1/500 s at f/5.6. This will give you a good amount of depth of field keeping the flower in focus as well as some of the area in front and behind it. If you wanted a larger amount in focus from front to back, you could change your aperture to f/8. This will constrict the amount of light coming into the sensor by 1 stop, which means you also need to increase the amount of time by 1 shutter speed. You will change it to 1/250 s. Both pictures will be properly exposed and can be considered good pictures. You could also go the other direction causing less depth of field. You need to adjust your f-stop to f/4 and then your shutter speed will go to 1/1000 s.
Having a camera that allows you to control both your shutter speed and aperture gives you much more creative control of your images.
You can use aperture priority mode in your camera to specifically control your depth of field.