Lesson 8: Aperture — Focal Point

Chelsea London © 2017. Fujifilm X-E1 | 56.0 mm | ƒ/2.2 | 1/1000s | ISO 200

Aperture

You’ll remember from previous lessons that the term "aperture" refers to the opening in a lens through which light passes to enter the sensor/film. It is typically a circular or hexagonal opening with an adjustable diameter. The size of the aperture is measured in f-stops or f-numbers, a ratio of the width of the aperture divided by the focal length of the lens. A smaller f-number (e.g., f/2.8) indicates a larger aperture opening, allowing more light to reach the camera sensor, while a larger f-number (e.g., f/16) represents a smaller aperture, letting in less light. While the numbering system may seem counterintuitive, remind yourself that we’re dealing in fractions.

The aperture plays a crucial role in controlling the amount of light that enters the camera, affecting the exposure of the image. Additionally, along with subject distance from the background, it influences the depth of field - the range of distances in the scene that appears in focus in the final photograph. Adjusting the aperture also contributes to creative effects such as background blur (bokeh) or maximizing the sharpness throughout the entire image.

Chelsea London © 2016. Fujifilm X-T10 | 16.0 mm | ƒ/18 | 6.5s | ISO 200

Depth of Field

Depth of field (DOF) is a term used in photography to describe the range of distances within a scene that appears in focus in a photograph. In other words, it is the area in front of and behind the main subject that appears to be in focus. A photograph with a shallow depth of field will have a limited portion of the image in sharp focus, while a deep depth of field will render a larger portion of the scene in focus.

Several factors influence the depth of field, with one of the primary ones being the aperture setting of the camera. A wider aperture (smaller f-number) results in a shallower depth of field, isolating the subject from the background and foreground by creating a pronounced blur in the out-of-focus areas. On the other hand, a narrower aperture (larger f-number) increases the depth of field, making more of the scene appear sharp and in focus.

Other factors that can affect depth of field include the distance between the camera and the subject, as well as the focal length of the lens. For this week, we’re going to concern ourselves mainly with the aperture’s effect on DOF.

Christian Fischer, CC BY-SA 3.0, via Wikimedia Commons

Diffraction

Diffraction refers to the bending of light waves as they pass through the aperture of a camera lens. This optical phenomenon becomes more noticeable at smaller aperture settings, which correspond to higher f-numbers. When light passes through a small aperture, it tends to spread out, causing the edges of the aperture to diffract the light.

In practical terms, diffraction can have an impact on image sharpness. As you stop down the aperture (use higher f-numbers), you might initially notice an increase in depth of field, but beyond a certain point, usually around f/11 to f/16 or smaller depending on the lens, diffraction starts to counteract the benefits. The diffracted light creates interference patterns that can lead to a loss of overall image sharpness.

Photographers often need to strike a balance between achieving the desired depth of field and avoiding the negative effects of diffraction. This is especially important in situations where maximizing sharpness is crucial, such as in macro photography or when using high-resolution camera sensors.

The consequence is that each lens has a sweet spot, an aperture at which its sharpness is optimal. The further you step away from this aperture, the worse the results will be. Depending on the general quality of the lens, it could be hardly noticeable, or it could ruin your images. The exact value of the sweet spot depends on each particular lens.