Tutorial - Aperture

Usually the term aperture is used interchangeably with the term "F-stop", however, technically they are different.

The aperture in a lens is the size of the hole that lets the light in to hit the sensor. Whereas the F-Stop is the ratio of the size of that hole compared to the focal length.

Assume the diameter of the hole letting in the light is 36mm and the focal length is 50mm. Then: Aperture = 36mm F-stop = focal length / diameter = 50mm / 36mm = 1.4

Some of the most common f-stops on lenses are 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22.

Right now, those numbers seem random but they are all approximate factors of the square root of 2.

Every time you change your f-stop from f1.4 to f2 or from f2 to f2.8 you halve the amount of light hitting the sensor. If you do not change any other settings, the picture will be darker. Adversely, if you go from f2 to f1.4 you double the amount of light hitting the sensor and the photo will be brighter.

The aperture can have a huge affect on the look of an image, the larger the aperture the shallower the depth of field.

Depth of Field

The depth of field is defined as the length of the in-focus portion of an image.

There are three things that affect depth of field in an image.

  • The size of the sensor

  • Size of the Aperture

  • Distance from the subject.

Since most people generally use the same camera body, the size of the sensor won't change between pictures. This factor only makes a difference when comparing the image between two camera bodies with different size sensors. For example APS-C, mirrorless or Full-Frame.

As can be seen in the example above, when it comes to aperture, the smaller the F-number = the larger the aperture. The larger the aperture also means a shorter the depth of field.

At F1.4, which means a large hole, you can have a person's eyes in focus while having their ears out of focus.

At 22, which means a small hole, you can have all the scenery from a few feet away all the way to infinity, in focus.

Aperture: F1.4

Two Jack Lake Out of Focus

A shallow depth of field. Notice how the mountain is out of focus.

Aperture: F22

Two Jack Lake in Focus

F22, Notice how both the rock in the foreground and the mountain are both in focus?

Likewise, the closer you are to your subject, the shallower the depth of field will be.

Depth of field is one of the most substantial ways to change an image. It can change the entire perception. In portraits or macro shots you want to isolate the subject from the background, whereas in landscape and astrophotography, you want everything in focus. Knowing how to get the look you want will open up your photography.

Next time we will go over Shutter Speed, ISO and how they fit together with aperture to give your photo a proper exposure.