If a camera is an eye, the aperture is its pupil. It is the hole through which light passes and is focused, and it is created and controlled by the iris surrounding it. In a camera’s case, the iris (or diaphragm) is made of several small metal blades overlaid in a circular pattern. Just as our pupils widen to let in more light in the dark, we can open our apertures in low light, and close them in brightness. What is less obvious, though, is the effect the aperture’s size has on depth of field.
What is Depth of Field?
A photo’s depth of field is the distance between the furthest and closest objects that are in focus. If both the foreground and the background of a scene are sharp, it has a large depth of field.
If a small area is in focus, but the front and back fall off into blur, it has a small or short depth of field. These are also known as deep focus and shallow focus.
Smaller Aperture, Sharper Image
The depth of field is controlled by the aperture. A large aperture, while it lets in more light, shortens and softens the focal plane (the precise distance at which your lens is focused). This is because the light comes in at steeper angles, causing it to scatter more. As the aperture shrinks, only the straight-on light rays can enter. This cuts down the total light, but sharpens the image and increases the depth of field.
The f-stop is a fraction, just like shutter speed, and so the smaller f-numbers designate larger openings. This means that apertures with values of f/2 and f/4 will make bright, soft-focus photographs, while f/16 and f/22 will create a large depth of field. However, the camera will need a longer shutter speed or higher ISO to compensate for the light loss.
Controlling the Background
Creative aperture control allows you to remove the background by blurring it out, which directs the focus to the subject. Using a shallow depth of field allows selective focus, which is used to highlight detail in a small area of a scene. Conversely, a large depth of field can bring the background into the picture, when needed.
A Bit About Bokeh
You have probably heard this word thrown around a lot, but what is bokeh? How do you even say it?
While the second question is a topic of endless (and fruitless) debate, most people agree that bokeh is the way the lens renders out-of-focus blur. This is most obvious in points of light; when defocused, the haze they create takes the shape of the aperture. Sometimes it is beautifully round, sometimes it’s octagonal, and can be more like a pentagon. Conventional opinion believes that the rounder the aperture, the better the bokeh.
Minimum, Optimum, and Maximum Apertures
Every lens has its limits; it will have a maximum f/stop – the largest its aperture will go – which we call wide open. The maximum aperture is noted in the name of a lens: “Nikon 50mm f/1.8”, for instance, or “Canon 200mm f/4”. On a zoom lens, this is often a range: “Pentax 18-55mm f/3.5-5.6”, where the maximum aperture is f/3.5 at 18mm and closes down to f/5.6 when zoomed to 55mm. Less advertised are the lens’ minimum apertures. They are often f/22 or f/32; only very specialized lenses will shrink down to f/64, or to Ansel Adams’ mythical f/90!
Every lens, too, has an optimum aperture. This is different for every lens, even different units of the same model. It is the aperture at which the lens achieves its sharpest focus at its center point; the point where all the lens elements line up as perfectly as possible, with no distortion. It is usually found around f/8, but can fall anywhere, depending on the lens design, and on chaos theory.
How to Use the Aperture
Large apertures are popular in portraiture and macro photography, to blur the background and focus on the main subject. They can also create a sense of depth, lead the eye, and separate the foreground and background. They’re used in any type of low-light photography, since they are arguably the most effective and dramatic way to let in more light without using a flash.
Small apertures are used in landscape photography, when the full depth of the scene is important and long exposures on a tripod are possible. For the same reason, they are preferred for architectural photography, both inside and out.
The aperture can be changed on a DSLR through either the Aperture-priority (“A”) or fully Manual (“M”) modes. Use a small f-number for a small depth of field and a large number for a large depth of field. On an automatic camera, such as the PicsArt camera app, use the “portrait” scene mode found in the menu (marked by three squares in a row) to get a large aperture and shallow focus. For a sharp image front to back, use the “landscape” mode, which closes the aperture down small.