Aperture size is the diameter of the opening inside a camera lens that controls how much light reaches the sensor. It works like the pupil of your eye: wider to let in more light, narrower to let in less. This opening is measured using f-stops (like f/2.8 or f/11), and it affects not just brightness but also how much of your photo appears sharp or blurry.
How the Aperture Works Physically
Inside every interchangeable lens, a set of thin, overlapping metal blades forms an adjustable opening called an iris diaphragm. These blades move together in a coordinated way, expanding or constricting to change the size of the hole light passes through. Because the opening is formed by flat-edged blades rather than a perfect circle, its shape is technically a polygon. A lens with more blades produces a rounder opening. Some lenses use 7 or 9 blades, while others designed for smoother background blur use 11 blades with curved edges to get as close to a true circle as possible.
Some lenses have a fixed aperture that cannot change. But most photography lenses let you adjust the aperture either through a ring on the lens barrel or through your camera’s controls.
F-Stops: How Aperture Size Is Measured
Aperture size is expressed as an f-number, sometimes written as f/2.8 or f/11. The f-number is a ratio: the lens’s focal length divided by the diameter of the aperture opening. So a 50mm lens set to f/2 has an aperture opening that’s 25mm wide. That same lens at f/8 has an opening just 6.25mm across.
This is why f-stop numbers feel counterintuitive at first. A smaller number means a larger opening, and a larger number means a smaller opening. F/1.4 is a wide hole that floods the sensor with light. F/22 is a tiny pinhole that lets very little through.
The standard f-stop scale runs: f/1, f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22, f/32. Each step in this sequence is called a “full stop.” Moving one full stop in either direction doubles or halves the amount of light hitting the sensor. The numbers in the sequence are each multiplied by roughly 1.4 (the square root of 2), because doubling the area of a circle requires increasing its diameter by that factor. Most cameras also let you adjust in half or third stops for finer control.
How Aperture Affects Exposure
The aperture’s most direct job is controlling brightness. Opening from f/8 to f/5.6 doubles the light. Opening again from f/5.6 to f/4 doubles it once more, meaning f/4 lets in four times as much light as f/8. This makes aperture one of the three pillars of exposure alongside shutter speed and ISO sensitivity.
In practical terms, a wider aperture lets you use faster shutter speeds (useful for freezing motion or shooting in dim light), while a narrower aperture may require slower shutter speeds or higher ISO to compensate for the reduced light.
How Aperture Controls Depth of Field
Beyond brightness, aperture size determines depth of field, which is how much of the scene appears sharp from front to back. A wide aperture like f/1.4 or f/2.8 produces a very shallow depth of field. Your subject might be razor-sharp while the background melts into a smooth blur. A narrow aperture like f/11 or f/16 keeps much more of the scene in focus, from objects close to the camera all the way to the horizon.
That creamy background blur, often called bokeh, is shaped by the aperture blades themselves. Lenses with more blades or curved blades produce rounder, smoother circles of light in out-of-focus areas. Lenses with fewer straight-edged blades produce bokeh with visible geometric shapes, like hexagons or pentagons. Whether this matters to you is partly a matter of taste. Some photographers find polygon-shaped bokeh perfectly fine, while others specifically seek out lenses designed for maximum roundness.
The Sharpness Sweet Spot
You might assume that stopping down to the smallest possible aperture gives you the sharpest image, but physics gets in the way. At very small apertures (high f-numbers like f/22 or f/32), light waves bend as they squeeze through the tiny opening, a phenomenon called diffraction. This bending actually softens the entire image. On the other end, wide-open apertures tend to show more optical flaws built into the lens design, like softness at the edges of the frame.
The result is that every lens has an optimum aperture where it’s sharpest, typically about 2 to 3 stops narrower than its widest setting. For many lenses, this sweet spot falls around f/8 to f/11. A lens that opens to f/2.8, for example, often delivers its crispest results somewhere around f/5.6 to f/8.
Common Aperture Settings by Situation
Portrait photographers typically shoot between f/1.4 and f/2.8 to isolate the subject from the background. The shallow depth of field draws attention to the person’s face while turning distracting backgrounds into a wash of color and soft shapes.
Landscape photographers generally work between f/8 and f/16. When there are objects both close to the camera and far away, something in the f/11 to f/16 range keeps everything acceptably sharp. If everything in the scene is roughly the same distance away, f/8 often gives the best combination of depth of field and overall sharpness. Going beyond f/16 or f/18 tends to soften the whole image from diffraction, which defeats the purpose of chasing front-to-back sharpness.
Street and general-purpose shooting often lands around f/5.6 to f/8, balancing enough depth of field to keep your subject sharp with enough light to maintain a comfortable shutter speed.
The Entrance Pupil vs. Physical Aperture
One subtlety worth knowing: the aperture opening you’d see if you physically removed the lens and measured the hole between the blades isn’t always what matters optically. What the camera “sees” is the entrance pupil, which is the image of the aperture as viewed through the front of the lens. Because the glass elements in front of the aperture can magnify or shrink its apparent size, the entrance pupil can be larger or smaller than the physical hole. This is why f-stop numbers remain consistent across different lens designs. The f-number is calculated using the entrance pupil diameter, not the raw physical opening, ensuring that f/8 on one lens delivers the same exposure as f/8 on another.
Aperture in Microscopes and Telescopes
Aperture size matters well beyond photography. In telescopes, the aperture is the diameter of the main mirror or front lens, and it determines how much light the telescope collects and how fine the detail it can resolve. Bigger aperture means fainter objects become visible and closer details can be separated.
In microscopy, the concept is expressed as numerical aperture (NA), which combines the half-angle of the cone of light entering the objective with the refractive index of the medium between the lens and the specimen (air, water, or oil). Higher numerical aperture means finer resolving power. Ernst Abbe formalized this relationship in the late 19th century: resolution improves as numerical aperture increases and as the wavelength of light decreases. Oil-immersion lenses achieve higher NA values than air lenses because oil has a refractive index of about 1.51 compared to 1.0 for air, effectively bending more light into the objective.
Across all these optical systems, the core principle is the same. Aperture size governs two things: how much light gets in and how finely the system can distinguish detail.