Lens Aperture

May 21, 2005


In photography, the term aperture refers to the hole inside a camera formed by the blades of a mechanical diaphragm, as demonstrated by the picture at right. Changing the aperture settings of a camera causes the blades to slide closer or farther apart, thus controlling the size of the opening.

The size of the aperture opening determines the amount of light that enters the camera after it has been reflected from the image we wish to photograph. Opening the aperture wider allows more light to enter the camera. Reducing the size of the aperture allows less light to enter. A camera's aperture is analogous to the pupil of an eye.

Aperture is expressed in terms of f-numbers (or f-stops) on my camera's LCD display. A small f-stop, such as F2, will mean a large aperture opening. A large f-stop, such as F16, will mean a small aperture opening. This may seem backwards, until you realize that f-numbers are actually the result of fractions, as explained below.

focal lengthF-numbers are the result of two measurements. One of the measurements, as shown in the picture to the left, is the distance (f) from the center of the camera lens to the focal point (F), which is the point at which the image comes into focus. This measurement (f) is known as the focal length of the image. The second measurement is simply the diameter of the aperture opening, usually expressed in millimeters.

The f-number is the result of the ratio of these two measurements, i.e., focal length to aperture diameter. A small f-stop, such as F2, means a relatively wide aperture opening, while a large f-stop, such as F16, means a relatively small aperture opening.

The aperture setting is important not only because it controls the brightness of a photograph, but because it also controls, to some extent, the focus of a picture. This is because of something called depth of field, which refers to the amount of a scene that will be in focus when the picture develops.

To demonstrate, I took the two photographs below. The photograph on the left was taken using the smallest aperture setting my camera would allow for the particular lighting conditions of this scene (F8). The photograph on the right was taken using the widest setting my camera would allow under the same conditions (F3.1).

F8 F3.1

Look closely and you will notice the photograph on the left contains a greater depth of field. That is, the background objects, such as the bag, are in focus as well as the foreground objects, such as the book. The photograph on the right demonstrates a shallower depth of field (only the book in the foreground is in focus).

So, perhaps a better way to look at the f-stops on my camera might be in terms of the depth of field I want to produce, rather than simply how light or dark I want to make the picture. Remember that a large f-stop produces a long depth of field while a small f-stop produces a shallow depth of field.

Using this knowledge, I can create different effects in my photographs. For instance, perhaps I want to create a portrait in which the subject remains clearly in focus while the surrounding objects appear diffused or softly out of focus. Or perhaps I want to create a wide panaroma in which everything stays in focus. Either of these effects can be achieved by manipulating the aperture setting before adjusting the shutter speed.

This type of photography is known as aperture-priority photography. It is the opposite of shutter-priority, in which the shutter speed is adjusted first and then the aperture is adjusted afterward. Shutter-priority can be useful in certain situations, such as when trying to capture objects clearly that are in motion, such as sporting events.

Thus, the relationship between shutter speed and aperture opening becomes apparent. It boils down to deciding what result you want and then adjusting the aperture setting and shutter speed accordingly to achieve the depth of field and amount of focus you desire.