When a photo is taken by a camera the light from the outside pass through an iris or a diaphragm before it reach the light sensitive sensor inside the camera body. The size of the opening in the iris is refered to as aperture and is expressed in f-numbers or f-stops. F-stop is then abbreviated as f/1.8, f/11, f/22 and so forth.
The higher the f-stop value, the smaller the opening in the iris become and a lower value then give a bigger opening. The effect this has on the photo taken is that the smaller the opening is, the bigger is the area in the photo which will be in focus. In the photo below a very small f-stop was used which resulted in a photo where there is only a small point on the top of the nail which is in focus while the rest of the photo is blurry.
In the photo below we can see the opposite use, a big f-stop, used to get the entire length of the photo to remain in focus. When we speak of focus in this manor in our photographs this is often referred to as the Depth of Field.
Aperture and shutter-speedis the two key factors which will help you get a photo with good exposure.
The camera is a closed device which stop all light from entering the body, the only part made to let light into the body and onto the film or light-sensitive electronic sensor is called the shutter. A number of different design exist but the purpose is the same even though the look can differ.
By selecting how long this shutter is open we can control how much light that we get into the sensor and by this decide the amount of light which will end up in the photo. To decide on how long we should keep the shutter open we have to think of two things. First of all, how much light is there around you where the photo is to be taken. If the photo is taken on a bright summer day we probable can keep the shutter open for a very short time but still get enough light onto the sensor to create a good looking photo. If there is not as much light surrounding the photographer, perhaps the photo is to be taken at dawn or in the middle of the night, we need to open up the shutter for a longer amount of time to let in enough light.
As that was not enough we also have to take in a second factor when it comes to shutter speed. If we are to take a photo of an object that is moving the slower shutter will give us a smeared look on the photo. When we take a photo with a slower shutter we can think of the shot as if we were actually filming instead and the photo we end up with is the merged photo of all the frames we took during the time the shutter was open. This is a great technique to take stunning photos of soft flowing water as seen in the photo below.
Photographs with a fast shutter is often refered to “frozen time”-photos because the look one achieve with a fast shutter is like the time froze and all details is preserved. This is often used when taking photos of sport events and such fast moving objects.
Sometimes having a shutter speed of up to a whole second wont be enough time to receive the amount of light to create a photo. To coop with this many newer cameras have an ability to keep the shutter open for a very long time, often refered to as bulb-photographing. Photos of nightskies often use this technique to have the shutter open for so long that light from the distant stars have time to reach the sensor in amounts to create a photo. Keeping the camera still without aid for many seconds, minutes or sometime even hours is not possible. A tripod is needed to do this.
So to conclude about shutter speed.
Shutter speed is measured in seconds. For example 1/100 as a hundredth of a second or 1/20 which then is a twentieths of a second. Note that 1/20 is a slower shutter then 1/100.