Camera aperture definition
Let’s start from the beginning. Aperture is the opening through which light travels. The width of the opening determines how much light can enter the lens and access the image plane. The camera aperture range refers to the amount that a particular lens can close or open to let in less or more light, and each separate measurement of aperture width is an aperture stop.
To make it a little simpler to understand, the mechanism works in a similar way to that of the human eye. Imagine for a moment that that the iris is the lens and the pupil is the aperture. In a dimly lit room, the pupil will expand to let in more light, and in bright daylight, the pupils will dilate, allowing less light to enter.
This phenomenon occurs in photography with aperture.
F-number and aperture stop
Take a look at the above diagram. These are a series of lenses depicting different aperture stops. Now, you’re probably wondering what the ƒ/? symbol means. The lens aperture is defined as an f-number, such as f/3.5 or f/22. Most cameras range from approximately f/2.8–f/22, but this varies greatly depending on the type of camera and lens that you own. Telephoto lenses, for example, have longer focal lengths, which create a relatively narrow aperture, whereas a wide angle lens has a relatively short focal length, allowing for a wider aperture.
The progression from the lower number to the higher number covers about six stops, as you can see from the diagram above. F-numbers are also called focal ratio, f-ratio, f-stop, or relative aperture. The f-number, specifically, is the ratio of the diameter of the entrance pupil (aperture) to the lens’s focal length. In the formula f/3.4, for example, the f symbol stands for focal length and the 3.4 is the diameter of the entrance pupil of the lens.
Now, this is where it gets tricky for most people. A higher f-number means a narrower aperture, so less light can access the image plane, and a lower F-number means a wider camera aperture, allowing more light entrance into the lens.
Take the above set of images, for example. The image on the left was taken with an f-number of ƒ/25, whereas the one on the right was taken with an f-number of ƒ/4. In the image on the left, less light has entered the lens, even though the f-number is much higher than the image on the right.
Now, what’s the first thing you notice about the two images? The cat in the left-hand photograph is relatively clear, while the one on the right is blurred, even though the plant is approximately the same level of sharpness in both images.
Aperture and depth of field
Why is this? Relative aperture affects something known as depth of field. Depth of field refers to the distance between the nearest and farthest objects in a frame that are considerably sharp in a photograph.
In the above photograph, the f-stop has been set to f/14, which is a relatively narrow aperture. Yet, you can easily see that the image is only sharp in the foreground. This is because the distance of the background is much farther from the lens than the flowers are and is, therefore, not within the depth of field.
Typically, the broader the depth of field, the more area of your photograph will appear sharp, and the smaller the depth of field, the smaller portion of your photograph will appear sharp. This is also relative compared to the distance your subject is from the lens.
Here’s another example. The above photograph was taken with an f-number of f/5.6, which is a mid-range to wide aperture. Yet, a large portion of the photograph remains blurred, because an f-number of f/5.6 is a shallow depth of field. Also, the subject of the photograph, in this case a squirrel, is far away from the lens. Because of this, the squirrel and the part of the wooden pole directly in front of him, are the only parts of the frame in focus.
As shown by the above illustration, a narrow depth of field captures only a very small portion of the sunflower, while the rest of it is blurred. In a wider depth of field, the entirety of the sunflower is in focus.
So, let’s review.
Aperture is the opening in a lens that allows light to enter.
The f-number or f-stop, represented by the formula f/number, is the ratio of the length of the focal lens to the diameter of the aperture or entrance pupil. Higher f-numbers mean narrower aperture, while lower f-numbers mean wider aperture.
Depth-of-field is the distance between the nearest and farthest objects that are relatively sharp in a photograph. Higher f-numbers mean greater depth of field, and vice versa.