F-stop (also known as focal ratio, f-ratio, or relative aperture) is a number that indicates the size of an aperture opening relative to the focal length of a lens. The f-stop is a fraction, such as Æ’/2.8, representing the ratio of the lens's focal length to the diameter of the aperture.
The lower the number, the larger the aperture, and the more light can pass through the lens. The higher the number, the smaller the aperture, and the less light can pass through the lens.
The f-stop also affects the depth of field of an image‚ or how much of the image is in focus. Lower f-stops create a shallow depth of field, while higher f-stops create a deeper depth of field. Let's take a look at some photos to see how f-stop affects depth of field:
Shallow Depth of Field (Æ’/2.8)
Notice how the subject (the pair of sunglasses) is perfectly focused while the background and foreground are blurred? That's because this photo was captured using a wide aperture (Æ’/2.8), creating a shallow depth of field.
Remember: the smaller the f-stop number, the wider the aperture, the more light touches the sensor, and the shallower the depth of field.
Deep Depth of Field (Æ’/8)
In this example, see how the entire frame is in focus. Using a smaller aperture, such as Æ’/8-Æ’/22, it is possible to focus on multiple scene elements, such as this beautiful lake and mountain in Yosemite National Park.
Remember: the higher the f-stop number, the smaller the aperture, the less light touches the sensor, and the deeper the depth of field.
Depth of Field: Explained
The most important part to remember about how aperture affects depth of field is not only the f-stop number itself but that and your distance from the subject itself. The closer the subject is to the lens, the shallower the depth of field will be. This is because the light entering the lens is spread over a shorter distance, resulting in a smaller image area appearing in focus.
Conversely, the further away the subject is from the lens, the greater the depth of field will be. This is because the light entering the lens is spread over a longer distance, resulting in a larger image area appearing in focus.
To put it in other words, a higher f-stop will result in a larger depth of field when the subject is far away (like in a landscape photo when using a wide-angle lens). In comparison, a lower f-stop will result in a shallower depth of field when the subject is close (for example, a photo of a flower using a 100mm macro lens with a wide aperture setting).
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