Photography Basics: Aperture

What is Aperture?

A device that controls the amount of light admitted through an opening. In photography and digital photography, aperture is the unit of measurement that defines the size of the opening in the lens that can be adjusted to control the amount of light reaching the film or digital sensor. The size of the aperture is measured in F-stop.

Thats a rather dry explanation for one of the Three Pillars of Photography (the other two being ISO and Shutter Speed). Simply put the aperture on a camera is the opening that controls the amount of light let through the lens on to the sensor/film of your camera. The wider the aperture the more light allowed through on to the sensor, and conversely the smaller the aperture the less light is let through. A good analogy for the aperture of a camera is to consider the human eye, in the human eye the pupil grows larger to let in more light and shrinks to reduce the amount of light allowed in, the pupil is the aperture of your eye. Varying the aperture on your camera can have dramatic effect on the image you shot. A wide aperture can give a nice out of focus background, while a narrow aperture can give a pin sharp image over the whole focal range.

So what is the aperture on a camera?

The aperture on a camera is a collection of plates inside the lens (often referred to as the diaphragm), as you make the aperture narrower these plates close together to reduce the size of the hole that the light is allowed through.

Aperture Diagram


The plates close together in a series of steps each step has an F number (also called an F Stop) such as F1.2, F11, etc. Just to confuse the matter slightly the lower the F number the wider the aperture and the more light is allowed in. So an aperture of F1.2 is wider than an aperture of F11.

Depth of Field

The reason that a smaller aperture effects how sharp the background of an image is because aperture has a direct impact on the depth of field of your image. A camera can only focus on one point at a time, however there is an area both in front and behind this point that appears sharp, this is the depth of field. Aperture is not the only thing than can effect it but is an important one (camera make and focal length also have an effect). The lower the aperture the shallower your depth of field will be and conversely the higher the aperture the deeper the depth of field will be. The two images below are a good example of how different apertures effect the image you create.

Large Depth of field

High aperture setting (F22) giving a deep depth of field

Shallow depth of field

Low aperture setting (F2.8) giving a shallow depth of field

The focal point of the image is the model airplane, and as you can see in the second image the foreground and the background are in soft focus isolating the plane from the rest of scene and making it stand out. In contrast, in the first image the whole scene is sharp and in focus, producing a clearly different image. As a photographer we want to use the settings on our equipment to create the effect we feel captures the moment the best. However it’s not just a matter of setting the aperture and taking the picture

When combined with the other two pillars of shutter speed and ISO, aperture controls the exposure of the image you capture, a photographer has to balance each one correctly to avoid using giving an under or over exposed image. At a lower aperture the shutter speed will need to be lower, while at a higher aperture you will need to up the shutter speed to allow longer for the light to get through the lens.

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