What Is Aperture? F-Stops, Light and Depth of Field

Aperture is the adjustable opening inside a lens that lets light through to the sensor. It is written as an f-number, which is the lens’s focal length divided by the diameter of the opening, so a smaller number means a bigger opening, more light and a shallower slice of the scene in focus.

Along with shutter speed and ISO, aperture is one of the three settings of the exposure triangle. Its side effect on the look of the picture, how much of the scene is sharp, is what makes it the setting people fuss over most. (Common misspellings you may have searched are “aperature” and “apature”. The word is aperture, from the Latin for an opening.)

How Aperture Works

Inside most lenses sits a diaphragm, a ring of overlapping blades that opens and closes to form a roughly circular hole. Light passes through that hole on its way to the sensor. Open the diaphragm wide and more light gets through in a given stretch of time. Close it down and less light gets through, the same way narrowing a tap slows the flow of water.

The size of the hole is written as an f-number: f/1.8, f/4, f/11 and so on. The f-number is the lens’s focal length divided by the diameter of its entrance pupil, the effective opening as seen from the front of the lens:

f-number = focal length ÷ aperture diameter

A 100mm lens with a 25mm-wide opening is shooting at f/4, because 100 divided by 25 is 4. The same lens with a 50mm-wide opening is shooting at f/2. This is why a bigger f-number means a smaller physical hole: for a fixed focal length, dividing by a larger diameter gives a smaller number. Almost everyone reads f-numbers backward at first: small number, big opening; big number, small opening.

Because the f-number is a ratio, it describes the same light-gathering ability on any lens. An f/2.8 aperture on a short lens and an f/2.8 aperture on a long lens deliver the same brightness at the sensor, even though the long lens has a physically larger opening. That is what lets you carry your exposure settings from one lens to another.

Aperture works the same way whether the light lands on film or a digital sensor. Older lenses had a ring on the barrel marked with the f-stops. Most modern lenses are set from a dial on the camera body, and the camera shows the current f-number on its screen.

The F-Stop Scale, and Why the Numbers Go That Way

Aperture is measured in stops, the same unit used for shutter speed and ISO. One stop always means a doubling or halving of light, whichever of the three settings produces it. The standard full-stop sequence is f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22. The table shows what each step does, using a 50mm lens as the example.

The numbers climb unevenly because the light passing through a circle depends on its area, not its diameter. To halve the area you shrink the diameter by a factor of the square root of 2, about 1.4. So each full stop is the previous f-number multiplied by about 1.4: 2 × 1.4 is 2.8, 2.8 × 1.4 is about 4, and so on. The sequence looks irregular, but every second number doubles (1.4, 2.8, 5.6, 11, 22), and so does every other one (2, 4, 8, 16).

Moving from f/4 to f/5.6 cuts the light in half, and back to f/4 doubles it. The step from f/16 to f/22 and the step from f/2 to f/2.8 are the same amount of light, even though one gap looks bigger on the page. Think in stops, not in raw numbers.

Still tripping over the f-numbers? Understanding F-Stops explains what an f-stop is, walks through the scale, and shows how each stop affects your photos.

Third stops

Most cameras also step in thirds of a stop between the full stops, which is why you see values like f/3.5, f/6.3 or f/9 on the screen. Between f/2.8 and f/4 the third-stop steps are f/2.8, f/3.2, f/3.5, f/4. Between f/4 and f/5.6 they are f/4, f/4.5, f/5, f/5.6. Between f/8 and f/11 they are f/8, f/9, f/10, f/11.

Very small apertures like f/32 or f/45 exist mainly on large-format, macro and specialty lenses. A typical zoom stops down to somewhere between f/16 and f/22. There is little reason to go further even when a lens allows it, because diffraction (below) softens the image and the shutter speeds needed become slow.

Aperture and Light

A wider aperture lets in more light, so it lets you use a faster shutter speed or a lower ISO in the same scene. Because a stop is a stop, you can trade one setting for another and keep the exposure the same. These all give the same brightness:

• f/2.8 at 1/500s
• f/4 at 1/250s
• f/5.6 at 1/125s
• f/8 at 1/60s

Each step closes the aperture one stop and doubles the exposure time to pay for it. The brightness stays put, but the picture changes: the depth of field grows with every step, and slower shutter speeds begin to show motion blur from a moving subject or a shaky hand. Opening up one stop does the opposite. It brightens the image or lets you halve the exposure time. That trade, light against sharpness against motion, is the whole exposure triangle in one line. In dim light it is often a wide aperture that makes a handheld photo possible.

Aperture and Depth of Field

The second job of aperture is depth of field: how much of the scene, from near to far, looks acceptably sharp. A wide aperture, a small f-number like f/1.8, gives a shallow depth of field. Your subject is sharp and what is in front of and behind it dissolves into soft blur. A narrow aperture, a large f-number like f/11, gives a deep depth of field, where a foreground rock and a distant ridge can both read as sharp in the same frame.

This makes aperture a compositional decision, not only an exposure control. Wide apertures isolate a subject and erase a distracting background. Narrow apertures hold an entire scene together from front to back.

Aperture is not the only thing that sets depth of field. Three other things matter as much:

• Distance to the subject. The closer you are, the shallower the sharp zone, at any aperture.
• Focal length. At the same distance and f-number, a longer focal length gives a shallower depth of field in the frame than a wide angle. Stepping back to keep the framing the same changes the picture, though, and the difference narrows.
• Sensor size. When you frame the subject the same way from the same spot, a larger sensor gives a shallower depth of field at the same f-number. A smaller sensor needs a smaller f-number to match: a sensor with a 1.5x crop factor at f/1.8 gives roughly the depth of field a full-frame sensor gives at f/2.8. The exposure is the same either way, since the f-number sets the brightness. Full frame and crop sensors are compared on their own page.

To see how thin the sharp zone really gets: a portrait at 2 meters on a full-frame camera with an 85mm lens at f/1.8 has a sharp zone only a few centimeters deep. Focus on the eye, and the ear can already be soft.

See What Aperture Does to Focus

Try it here. Pick an aperture and a focal length, tap a person to focus on them, and watch how much of the row stays sharp.

Want everything sharp from the foreground to the horizon? Hyperfocal Distance explains what it is, how to calculate it, and how to use it for the most depth of field.

See It Side by Side

Which photo has a shallower depth of field?

A B

Aperture Examples: Comparing Real Photos

These photographs use very different apertures. The flower at the top of the page was shot at f/2.8. Watch what happens to everything that is not the main subject as the f-number climbs.

What f/2.8 and Other Common Apertures Look Like

Each common setting has a typical use. These are habits, not rules.

• f/1.4 to f/2. The widest apertures. Very shallow focus, the most light, and a smooth wash behind a single subject. Useful in dim rooms and for isolating a face. Expect a sharp zone that can be thinner than the subject.
• f/2.8. A workhorse. It still blurs a background well, it gives a fast shutter speed indoors and at dusk, and it is deep enough that most of a face is sharp at portrait distances. Zoom lenses with a constant f/2.8 are common for events and sports for this reason. Two people side by side at the same distance are usually both sharp at f/2.8.
• f/4. A balance of blur and safety. The background is still soft, but small focus errors forgive themselves. A good choice for a couple, a pet, or a close object with some depth.
• f/5.6 to f/8. Enough depth for small groups and most street scenes, and usually where a lens is at its sharpest.
• f/8 to f/11. The landscape range. A foreground and a distant horizon can both be sharp when you focus a little way into the scene, as a rough guide a third of the way in, or use the hyperfocal distance.
• f/16 and beyond. Maximum depth of field for close-up and macro work, or a long exposure on a bright day. Diffraction begins to cost fine detail.

Bokeh and the Shape of the Blur

The quality of the blur behind a subject is called bokeh. A wide aperture produces more of it, but the look depends on the lens as well. The diaphragm that forms the opening has a number of blades. More blades, or curved blades, give rounder out-of-focus highlights. Fewer straight blades give angular shapes, visible as polygons in bright points like string lights. Stop the lens down and the highlights take on the shape of the blades more clearly; wide open they are circles.

More blur needs more than a wide aperture. It also needs a subject that is close to you and a background that is far behind the subject. A flower at arm’s length with a distant background blurs far more than a person standing right in front of a wall, even at the same f-number.

Sharpness, the Sweet Spot and Diffraction

Most lenses are at their sharpest one to three stops narrower than their widest opening. A lens with a maximum of f/1.8 is often sharpest around f/4 to f/5.6, and one with a maximum of f/2.8 around f/5.6 to f/8. Wide open, lenses show more optical flaws such as soft corners and color fringing. Stopping down cleans most of these up.

Closing the aperture too far backfires. Past the sweet spot, diffraction bends the light as it squeezes through the small opening and spreads each point of detail into a blur. It grows as the aperture shrinks. Where it becomes visible depends on pixel size: on a high-resolution full-frame camera it can start to cost fine detail around f/8 to f/11, and it gets worse toward f/22. Smaller sensors have smaller pixels, so it shows up sooner, often a stop or two earlier. That is why many landscape photographers settle around f/8 to f/11: narrow enough for depth, not so narrow that diffraction erases the gain.

Want to find where your own lens is sharpest? How to Test a Lens for Sharpness and Defects shows how to shoot a series and compare the centre and corners at every aperture.

Sharpness also depends on focus and shutter speed, not only aperture. A very wide aperture magnifies focus errors, and a very narrow one often forces a shutter speed slow enough for camera shake to blur the frame. See how to take sharp photos for the full set of causes.

Maximum Aperture, Fast Lenses and Variable Aperture Zooms

Every lens has a widest aperture it can open to, its maximum aperture, printed on the barrel: 50mm f/1.8, 24-70mm f/2.8. A lens with a small maximum f-number is called “fast” because the wide opening lets you use a faster shutter speed in the same light. Fast lenses need larger, more precisely shaped glass elements, so they are usually larger and heavier.

Many zoom lenses have a variable aperture, written as a range such as f/3.5-5.6. The widest f-number available changes as you zoom: such a lens opens to f/3.5 at its wide end and only to f/5.6 at its telephoto end. Zoom in and the image darkens by about a stop and a third, and in aperture priority the shutter speed slows to compensate. A constant-aperture zoom, such as f/2.8 throughout, holds the same light-gathering at every focal length.

If you are weighing a fast fixed focal length against a zoom, prime vs zoom lenses compares the aperture advantage of primes with the flexibility of zooms.

Using Aperture Priority Mode

Aperture priority, marked A or Av on the mode dial, is the quickest way to think in terms of depth of field instead of exposure numbers. You choose the aperture, the camera measures the scene and picks a matching shutter speed, and ISO can stay on Auto within a range you set. You dial in how much of the scene should be sharp and move on.

It is a real working mode for most photography, not only a stepping stone, and it suits any shoot where the light keeps changing. Watch the shutter speed the camera picks. If you choose f/11 in dim light, it may select a speed too slow to hold by hand. Widen the aperture, raise the ISO, or use a support. If the image comes out too bright or too dark, exposure compensation shifts it without changing the aperture.

Ready to set it up on your camera? Aperture Priority Mode explains how the mode works, when to use it, and how exposure compensation and Auto ISO fit in.

Choosing an Aperture by Situation

Start with the question “how much of this should be sharp?” and then pick the aperture that gives it.

• Single portrait. f/1.4 to f/4. The lower end melts the background into a smooth wash. Focus on the nearest eye.
• Groups. It depends on how far apart in depth the people are, not on how many there are. A row of people standing at the same distance sits in one plane, and f/4 may be enough. People staggered front to back need f/5.6 to f/8. Focus on a face in the front rows and check the back.
• Landscapes. f/8 to f/13 for sharpness across the frame, while staying clear of the f/22 end where diffraction bites. For more control, see landscape photography.
• Action and sports. Wide, around f/2.8 to f/4, for two reasons: it separates the athlete from a busy background and it permits the fast shutter speed that freezes motion. See sports photography.
• Close-ups and macro. Narrower than you expect, often f/8 to f/16, because focusing close makes the sharp zone very thin. See macro photography.
• Street and travel. f/5.6 to f/8 gives a forgiving sharp zone when things move quickly and you cannot refocus.
• Architecture. f/8 to f/11 for sharp lines and detail from near to far. See architectural photography.
• Video. Aperture controls depth of field in video just as in photos. Shallow focus looks cinematic but is harder to hold on a moving subject.

Try It Yourself: Camera Simulator

Open the aperture wide to throw the background out of focus, then close it down and watch the scene sharpen from front to back. The challenge below asks you to hit a target depth of field, the same decision you make on a real camera.

Want the other challenges? The full camera simulator has all 21, across eight scenes.

Want to learn aperture alongside shutter speed and ISO, one lesson at a time? Fundamentals Lesson 3: Using Your Camera’s Settings puts all three together, with a quiz at the end.

Common Mistakes

• Shooting wide open by default. f/1.4 looks dramatic, but on a close subject the in-focus zone can be a few millimeters thick: an eye can be sharp while the tip of the nose already is not. Stop down a little for any subject with real depth.
• Assuming the smallest aperture is the sharpest. Past a lens’s sweet spot, diffraction softens the image again. f/22 is rarely the sharpest setting a lens can produce.
• Ignoring distance and focal length. A telephoto lens at a wide aperture has an extremely thin sharp zone even at moderate distances, and moving closer thins it further. Focus precision matters more as reach increases.

Try This: Ten Minutes with One Subject

Find a single subject with something clearly in front of it and something clearly behind it, such as a plant on a windowsill with a cluttered room beyond. Set aperture priority, put your camera on a stack of books or a tripod, and photograph the same frame at your lens’s widest aperture, then at f/5.6, then at f/16. Let the camera change the shutter speed. Compare the three on a screen: how much of the room reads as sharp at each setting, how the blur changes shape, and whether the f/16 frame looks slightly softer overall. Then pick the one you would keep and note which aperture you chose, and why.

Aperture FAQ

What does aperture mean in photography?

Aperture is the size of the opening in the lens that lets light reach the sensor, written as an f-number such as f/2.8. A smaller number means a larger opening, more light and a shallower depth of field.

What does f/2.8 mean?

It means the opening’s diameter is the focal length divided by 2.8. On a 50mm lens that is about 18mm. It is a wide aperture that lets in four times the light of f/5.6 and gives a clearly blurred background on a close subject.

Which aperture should I use for sharp photos?

For the sharpest result from the lens itself, use its sweet spot, typically f/5.6 to f/8. For depth across a landscape, f/8 to f/11. “Sharp” depends on your goal: a portrait at f/1.8 is sharp where it matters, the eyes, even though the background is blurred on purpose.

Why are my wide-aperture photos soft?

Three things usually. The sharp zone is so thin that a small focus error shows; most lenses are optically softer wide open; and a shutter speed that is too slow lets motion blur creep in. Stop down one stop and check your shutter speed.

If you want to test yourself, try the Photography Exposure Quiz or the Aperture and Depth of Field Quiz.