The Aperture is one of the main settings in photography. In this section we will be looking at what the aperture controls and how the aperture can affect the end image. Many newer cameras come with an aperture priority mode which is a great way to learn how the aperture can change the look and feel of an image.
Understanding Aperture priority
To understand how Aperture priority works, it is crucial to understand what the aperture is, what it controls as well as how to adjust it. The aperture controls two main functions; the first function allows the photographer to control how much light can pass through the lens. The second function of an aperture is to control the depth of field of an image.
It is important to understand how the aperture controls light; the aperture controls light by un-constricting or constricting the light that passes through an opening in a lens more commonly known as the iris or diaphragm. The diaphragm is comprised of overlapping metal blades that open and close to change the size of the opening; the changing of the diaphragm size is commonly referred to as a f-stop.
The aperture allows a photographer to let more light into the camera’s lens when there is insufficient light in a scene. In contrast if the photographer finds that there is an abundance of light in a scene the aperture can be used to block light from entering the lens by merely changing the size of the aperture thus changing the amount of light that can enter the camera body.
The second function of the aperture is to control the depth of field in an image. The depth of field refers to the area of acceptable focus or the acceptable amount of blur in an image. This function is used by photographers to isolate their subjects from the background.
To see a diagram that shows the most common Aperture sizes Click here.
When placing a digital camera in aperture mode (A on Nikon; A.V. on Canon), the camera will attempt to balance the exposure with the ISO and shutter speed while the photographer chooses the size of the diaphragm. By placing the camera in aperture mode any photographer can learn how each f-stop will affect the end image without having to know what the other settings are. However, it is the photographer's responsibility to understand how to read the amount of light falling on and around the subject.
The F-stop or F-number controls two things the amount of light allowed to enter the camera lens and the depth of field this is controlled by changing the diaphragm's size. It is important to note that each time the diaphragm size changes, the amount of light that is allowed into the lens or is blocked from the lens is equal to double the amount of light.
When going from a larger diaphragm to a smaller diaphragm, the aperture allows less light through the lens; Photographers refer to this as stopping the aperture down. By stopping the aperture down two things happen the amount of light is reduced and the depth of field changes.
There are many reasons that a photographer wants to open a lens. One of the main reasons is so the subject can become isolated from the background. This means that the subject is sharp, and the background is blurry. The most common apertures sizes to achieve this is F/1.8, F/2.0 F/2.8 F/4.0. While having the camera in aperture mode the camera will choose the best setting for the shutter and ISO.
The next reason the photographer may want to open a lens up is to allow more light into the camera body. This can be a result of insufficient light in a scene, the ability to allow more light into a lens, allows the camera’s programming to choose a faster shutter speed and a lower ISO allowing for the best image possible When going from a larger diaphragm to a smaller diaphragm, the aperture lets less light in the lens; photographers refer to this as closing the aperture. When the size of the diaphragm is smaller there will be less light entering the camera making the exposure darker, yet while in aperture mode the camera will pick the best setting to insure the best exposure.This change in diaphragm size also changes the depth of field, as the diaphragm get smaller the light is constricted and makes the depth of field greater allowing more of the image to become sharper.
The below diagram shows the aperture sizes and their respected F-stop numbers. Notice that the smaller the diaphragm size, the larger the F-stop number. Equally, the larger the diaphragm size, the smaller the F-stop number. Keep in mind that when using a smaller diaphragm, the Aperture lets in less light through the lens which means there should be a lot of light in and around the subject. When using a larger diaphragm, the lens lets in more light, which means there is less light falling on and around the subject.
Another characteristic of the Aperture is the ability to isolate the subject from the background or bring the entire image into focus. The change in focus is called the depth-of-field or D.O.F. The depth-of-field refers to the amount of acceptable focus in the field of focus of the image. In other words, the sharp or the acceptable amount of blur in the image. The diagram to the left shows what happens to the field of focus in a few F-stops.
When using a large aperture number like F-22 (meaning a small diaphragm), the field of focus will cover the most significant distance in the image. This is also called deep focus or long depth-of-field (G.D.O.F.), the distance between the nearest and farthest objects in a scene that appears acceptably sharp in an image. G.D.O.F. is mostly used in landscape photography. When using this kind of D.O.F, there are a few factors that need to be considered. There needs to be a lot of light on and around the subject, and the image should take into consideration the background and the foreground. If these are not considered, the image may have distracting elements.
When using a small aperture number like F-2.8 (meaning a large-diaphragm), the field of focus will cover a lot less distance in the image. This is called shallow depth-of-field (S.D.O.F.), which is used primarily to isolate the subject from its environment. S.D.O.F. is mostly used in portrait work, macro photography, sports photography, and the arts. When using an S.D.O.F, a few factors should be considered: The subject will be isolated from the background due to the S.D.O.F. The exposure time may vary depending on the amount of light on or around the subject. You do not need a lot of light to make your exposure, the use of shutter speed, and the ISO will balance the exposure time.
When using a medium-size aperture number like F-8 (meaning a medium diaphragm), the field of focus will cover less than the G.D.O.F. and more than the S.D.O.F. in the image. This is called medium or journalistic depth-of-field. It is used primarily to show most of the image in sharp focus yet still allowing for some blur. This is mainly used in journalistic and event photography. When using this kind of D.O.F, there are a few factors that should be considered: there needs to be enough light on and around the subject. The image should also take into consideration the background as well as the foreground. If these are not taken into account, the image may have distracting elements.
The second way to change your depth-of-field is to simply change the distance between the lens and the subject. The closer the lens is to the subject, the smaller the field of focus is in the lens. This is because the closer you are to the subject, the harder it is for the lens to focus. This is called minimum focus distance, which is the shortest distance at which a lens can focus. Likewise, the closer the lens is to the subject, the blurrier the background will tend to be. The Below diagram shows the minimum focus distance.