Aperture
Written by Abin Alex and Anu Joseph.
Abstract
Aperture is the opening in a lens that sets how much light reaches the sensor. It is written as an f-number, the focal length divided by the width of the opening, so a small number means a wide opening, more light and a shallower depth of field. This guide explains the diaphragm, absolute and relative aperture, the full-stop steps of the f-number scale and their half and third divisions, effective aperture and lens speed. It relates aperture to exposure and to depth of field. It is written for photographers and cinematographers who want to choose an aperture on purpose.
Keywords: aperture; f-number; f-stop; lens speed; depth of field
- What is aperture, and what does it do?
- What is a diaphragm, and how does it work?
- What is the difference between absolute and relative aperture?
- How is the f-number written, and why does a bigger number mean a smaller opening?
- Why does each f-stop multiply by 1.4 and not by 2?
- What are full, half and third stops of aperture?
- What is effective aperture?
- What is lens speed, and what does maximum aperture mean?
- How do I set the aperture on my camera?
- What is the difference between an f-stop and a T-stop?
- Does a smaller aperture always make a sharper picture?
- How does aperture affect depth of field?
- How do cinematographers use aperture?
- Which aperture suits each kind of photography?
What is aperture, and what does it do?
Short answer. Aperture is the opening in the lens that lets light through to the sensor. It controls two things at once: how bright the picture is, and how much of the scene looks sharp.
- The opening sits inside the lens, formed by a ring of blades called the diaphragm.
- A wider opening passes more light, so the picture is brighter and the sharp zone is thinner.
- A narrower opening passes less light, so the picture is darker and the sharp zone is deeper.
- Each full step, called a stop, doubles or halves the light.
Light that the lens has focused must pass through a round diaphragm before it is recorded on the sensor. The opening in that diaphragm is the aperture. It does the same job as the pupil of the human eye, and the diaphragm is the camera version of the iris. It blocks every ray except those that go through the opening.
Figure 1. The aperture does for the camera what the pupil does for the eye.
Cham tipThink of aperture as a window. A wider window lets in more light and gives a softer background.
Cham taskShoot one subject at your lens's widest aperture and again at f/8. Compare the brightness and the background blur.
What is a diaphragm, and how does it work?
Short answer. The diaphragm is a ring of thin, overlapping metal blades inside the lens. The blades open and close to form the aperture. The design has progressed from loose metal plates to a motor-driven iris.
- Waterhouse stop: a metal plate with a hole, slid into a slot in the lens barrel. Each plate gave one fixed opening. It was invented by John Waterhouse in 1858 (Präkel, 2009). Sets of these plates were still sold in the late nineteenth century.
- Rotating diaphragm: a disc with holes of different sizes, turned to bring the chosen hole in front of the lens.
- Iris: tissue-thin leaves that overlap like flower petals and sit between two lens elements.
- Click stops and control wheel: a ring on the lens clicked at each full stop. On modern cameras an electronic wheel replaces the ring, and a motor moves the iris.
- Blade count: most current lenses have about 5 to 10 blades. Straight blades make a polygon-shaped opening and curved blades make a rounder one.
Today the aperture ring, or the control wheel on a digital camera, still drives a multi-bladed iris, but through motors. When you press the shutter button, the camera reads the exposure settings and signals a small motor in the lens, which turns the ring that moves the blades to an exact angle. Automatic diaphragms came first on SLR cameras: the lens stays wide open while you compose, so the viewfinder is bright, and closes down only during the exposure.
Cham tipIf your lens has an aperture ring, turn it and watch the blades change the opening through the front of the lens.
Cham taskFind how many aperture blades your lens has and note whether the opening looks round or angular.
What is the difference between absolute and relative aperture?
Short answer. Absolute aperture is the real width of the opening in millimetres. Relative aperture compares that width with the focal length and is what we call the f-number.
- Absolute aperture: the diameter, for example 50 mm.
- Relative aperture: focal length divided by diameter, for example 100 mm ÷ 50 mm = f/2. It has no unit.
- The same f-number gives the same brightness on lenses of any focal length, which is why exposure settings carry over when you change lenses.
- Lens barrels sometimes print it as a ratio, such as 1:3.5 to 5.6, which means f/3.5 to f/5.6.
One limit: the marked f-number is exact only when the lens is focused at infinity. At close focus, as in macro work, less light reaches the sensor than the marked number suggests, and photographers use a larger working f-number. The standard itself applies only to lenses focused at infinity (International Organization for Standardization, 2008).
Cham tipAbsolute aperture is the real hole. Relative aperture is the f-number. Compare lenses by f-number.
Cham taskWrite the focal length and maximum f-number of two lenses. Divide focal length by f-number to find the opening size of each.
How is the f-number written, and why does a bigger number mean a smaller opening?
Short answer. Write it as f/2.8: a lowercase f, a slash, then the number. A bigger number means a smaller opening, because the number is the divisor.
- f/2 means focal length ÷ 2, so the opening is half the focal length wide. f/16 means focal length ÷ 16, a much smaller hole.
- The slash shows the division. F2 hides it, which is why the f/ form is preferred.
- The same quantity is also called f-stop, f-ratio, focal ratio or just f-number.
- An easy memory: f/1.8 is like 1/1.8, a bigger fraction than f/8, which is like 1/8.
If italics are not available, f/2 is acceptable. Mathematically f/2 is the same as f ÷ 2 or 0.5f, but photographers, manufacturers and cameras do not write it that way.
Cham tipA small f-number means a big opening. Read f/2 as one half and f/8 as one eighth.
Cham taskWithout looking, say which opening is largest: f/1.8, f/4 or f/16. Then check it on your lens.
A 100 mm lens is set to f/4. How wide is the opening?
Show the answer
25 mm
Why does each f-stop multiply by 1.4 and not by 2?
Short answer. The light that gets through follows the area of the opening, and area depends on the square of the diameter. To double the light, the diameter only needs to grow by the square root of 2, about 1.4.
- One stop means double or half the light.
- Area = π × (D ÷ 2)², so double the area needs D × 1.414.
- That is why the full-stop series is 1, 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22.
- The diameter never doubles or halves from one stop to the next.
| Step | Working |
|---|---|
| Circle 1 | D1 = 20 mm. Area = 3.14 × 10² = 314 |
| Circle 2 | One stop brighter, so double the light: 2 × 314 = 628 |
| Compare | (D1 ÷ D2)² = 314 ÷ 628 = 1/2, so D2 = D1 × √2 |
| Result | D2 = 20 × 1.414 = 28.3 mm, about 28 mm |
Cham tipEach full stop multiplies the f-number by about 1.4 because light depends on the area of the opening, not its width.
Cham taskWrite the full-stop series from f/1 to f/22 from memory, then check it against your lens.
You close from f/5.6 to f/11 and keep the same brightness. By what factor must the exposure time grow?
Show the answer
four times, as the light falls by two stops
What are full, half and third stops of aperture?
Short answer. A full stop doubles or halves the light. Cameras also divide a stop into thirds, and some into halves, for finer control of exposure. The numbers printed on the lens are rounded.
- Exact factors between steps: full stop × 1.414, half stop × 1.189, third stop × 1.122.
- The printed numbers are rounded. The true values are 5.66, 11.3 and 22.6, printed as 5.6, 11 and 22.
- Every second number in the series is double the one two places back: 2, 4, 8, 16, 32.
- Third-stop steps are the ones most cameras use today.
| Full stop | One third of a stop smaller opening | Two thirds of a stop smaller |
|---|---|---|
| 1 | 1.1 | 1.2 |
| 1.4 | 1.6 | 1.8 |
| 2 | 2.2 | 2.5 |
| 2.8 | 3.2 | 3.5 |
| 4 | 4.5 | 5.0 |
| 5.6 | 6.3 | 7.1 |
| 8 | 9 | 10 |
| 11 | 13 | 14 |
| 16 | 18 | 20 |
| 22 | 25 | 29 |
| 32 | 36 | 40 |
Table 1. Third-stop steps after each full stop, as marked on cameras. Calculated as the full stop × 1.122 and × 1.26, then rounded.
Cham tipUse third stops for fine exposure changes and full stops when you want a clear change.
Cham taskCount the clicks from f/4 to f/8 with your camera set to third stops. Then count them in full stops.
Which lets in more light, f/2.8 or f/8, and by how many stops?
Show the answer
f/2.8, by three stops: 2.8, 4, 5.6, 8
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What is effective aperture?
Short answer. The effective aperture, or entrance pupil, is how wide the opening looks when you view it from the front of the lens. The f-number is the focal length divided by this apparent width, not by the physical hole in the blades.
- The glass in front of the diaphragm magnifies or reduces the opening, so the apparent opening is not the same as the real one.
- The entrance pupil is the image of the aperture stop as seen from the subject side.
- That is why you cannot get the f-number by measuring the blades with a ruler.
- The standard defines the f-number this way, using the entrance pupil.
Cham tipThe f-number is marked for a distant subject. Very close up, less light reaches the sensor than the f-number suggests.
Cham taskShoot a subject far away and then at your lens's closest focus, with the same settings. Compare the brightness.
What is lens speed, and what does maximum aperture mean?
Short answer. The widest opening a lens can reach is its maximum aperture, also called its speed. A lens that opens to f/2.8 or wider is called a fast lens, because it gathers more light and permits a faster shutter. A lens that opens only to f/4 or narrower is called a slow lens, because in the same light it needs a slower shutter or a higher ISO.
- The maximum aperture is printed in every lens specification, for example 50 mm f/1.8.
- Fast lens versus slow lens: a 50 mm f/1.8 prime is fast, and a typical 18 to 55 mm kit zoom, f/3.5 to 5.6, is slow. In dim light the slow lens struggles first.
- The minimum aperture matters less. Most lenses close to f/16 or f/22, and some to f/32.
- Prime lenses have one focal length and often open wider than zooms.
- Zooms can be variable aperture (such as f/3.5 to 5.6, which gets slower as you zoom in) or constant aperture (such as 70 to 200 mm f/2.8, which stays f/2.8 across the range).
Cham tipA fast lens lets in more light, so you can keep a quicker shutter speed in dim places.
Cham taskRead the maximum aperture printed on your lens. List two situations where it helps.
How do I set the aperture on my camera?
Short answer. Aperture priority mode (A on Nikon, Sony and Fujifilm, Av for Aperture Value on Canon) is a semi-automatic mode. You choose the aperture and the ISO, and the camera picks a matching shutter speed (Canon, n.d.). Use manual mode to set all three yourself.
- Aperture priority mode is semi-automatic: you choose the aperture and the ISO, and the camera calculates the matching shutter speed for a balanced exposure.
- It is the simplest way to control depth of field: choose a wide opening for a blurred background, or a narrow one to keep more of the scene sharp.
- Manual: you set the f-number, shutter speed and ISO.
- Turn the control dial to change the f-number. Each click is usually a third of a stop.
- When you close the aperture, the camera slows the shutter to compensate. Watch for blur from camera shake.
Cham tipWhile learning, use Aperture Priority (A or Av). You choose the f-number and the camera picks the shutter speed.
Cham taskIn A or Av, shoot at the widest aperture, f/5.6 and f/11. Note the shutter speed the camera chooses each time.
What is the difference between an f-stop and a T-stop?
Short answer. An f-stop is calculated from the size of the opening. A T-stop is the f-number corrected for the light that the lens glass absorbs, so it tells you the light that really reaches the sensor. A T-stop is always larger than the f-number.
- Real lenses transmit 60% to 95% of the light, because some is lost inside the glass (PetaPixel, n.d.).
- T = f-number ÷ √transmittance.
- Two lenses at the same f-stop can have different T-stops. The Zeiss Otus 55 mm and 85 mm are both f/1.4, but measure about T1.5 and T1.7 (PetaPixel, n.d.).
- Light-meter readings, depth-of-field tables and EV charts all use f-stops.
| Lens | f-stop | Transmittance | T-stop |
|---|---|---|---|
| Any lens (example) | f/2 | 75% | T2.3 |
| 50 mm prime (example) | f/2.8 | 75% | T3.2 |
| 70 to 200 mm zoom (example) | f/2.8 | 70% | T3.3 |
The percentages are teaching examples. A zoom has more glass than a prime, so it usually loses more light.
Cham tipUse T-stops to match exposure between different cine lenses, because they count the light lost inside the glass.
Cham taskLook up one cine lens. Write its maximum f-stop and its T-stop, and see which number is larger.
A lens marked f/2.8 transmits 70% of the light. What is its T-stop?
Show the answer
about T3.3
Does a smaller aperture always make a sharper picture?
Short answer. No. A very small opening bends light at its edge and softens the picture, and a very wide one shows the flaws of the lens. Most lenses are sharpest a few stops below their widest opening, around the middle of their range.
- Diffraction: light spreads as it passes a small opening, so each point becomes a small disc. The disc is about 2.44 × wavelength × f-number wide (Edmund Optics, n.d.). For green light (0.55 micrometres) at f/16 that is about 21 micrometres, several times the width of a typical camera pixel, so the blur spreads over neighbouring pixels.
- Wide open: lens flaws called aberrations are strongest at the widest openings.
- Sweet spot: most lenses are sharpest a few stops down from the widest opening, around the middle of their range.
- Test your own lenses, because every lens is different.
Cham tipMost lenses are sharpest a couple of stops below wide open. Very small apertures lose sharpness to diffraction.
Cham taskOn a tripod, shoot a page of text at f/2, f/5.6, f/11 and f/22. Compare them at 100 percent.
How does aperture affect depth of field?
Short answer. A wider aperture (smaller f-number) gives a shallower depth of field. A narrower aperture (larger f-number) gives a deeper one.
- Depth of field is the range of distances that looks acceptably sharp.
- Aperture is one of four factors. The others are subject distance, focal length and sensor size.
- Changing aperture also changes exposure, so balance it with shutter speed or ISO.
Cham tipFor a blurry background, open the aperture, move closer to your subject and keep the background far behind.
Cham taskPhotograph a friend with a distant background at f/2 and f/11 from the same spot. Compare the backgrounds.
How do cinematographers use aperture?
Short answer. A DOP picks the T-stop for the look first, because it decides how thin the focus is. Exposure is then set with lights, ND filters and sensor sensitivity, since the shutter speed is tied to the frame rate.
- T-stops are used so that every lens in the set exposes the same.
- ND filters cut the light so the aperture can stay open for the look you want.
- Changing the iris during a shot changes the exposure on screen, so it is done carefully and only when the shot calls for it.
- The focus puller, a member of the camera crew, keeps the subject sharp inside the depth of field the T-stop gives.
- On Indian and Hollywood sets the language is the same. The cinematographer is called the DOP in India and the DP or cinematographer in Hollywood, and the focus puller is the 1st AC (first assistant camera).
- Opening up means widening the aperture, and stopping down means closing it. Shooting wide open means working at the lens's widest T-stop.
- Splitting the stop means setting a value between two marks, for example "T2.8 and a half", which lies between T2.8 and T4.
- An iris pull, or ramp, is a change of aperture during a shot, for example to hold exposure as the light changes from day to dusk. Cine lenses have a de-clicked aperture ring so the change is smooth.
- Deep focus keeps near and far both sharp, using a small aperture and a lot of light. Gregg Toland shot nearly all the interiors of Citizen Kane (1941) at f/8 or narrower and raised the light to compensate (Toland, 1941). Shallow focus is the opposite, with a wide aperture and a thin focus plane.
- Fast glass is a common name on set for wide-aperture lenses, such as T1.3 to T2 cine primes.
Cham tipCinematographers often choose the aperture for the look first, then balance exposure with ND filters or light.
Cham taskFilm a person at the widest aperture and again at f/8, using an ND filter or lower ISO to hold exposure. Compare the focus fall-off.
Which aperture suits each kind of photography?
Short answer. Use a wide aperture (small f-number) for a soft background or more light, and a narrow aperture (large f-number) when more of the scene must be sharp. Treat every line below as a starting point. The light and the lens will change it.
- Birds. Wide open, usually f/4 to f/5.6 on a long lens. Stop down to f/8 when the light allows, for sharper feathers.
- Animals and pets. f/2.8 to f/4 to separate the animal from the background. Use f/5.6 when the whole face must be sharp.
- Reptiles. f/5.6 to f/8. At close range a wide aperture keeps only the eye sharp.
- Insects and macro. f/8 to f/16. The depth of field is only millimetres, so add focus stacking. Avoid going far beyond f/16, where diffraction softens the picture.
- Portraits. f/1.8 to f/2.8 for a soft background. Use f/4 for two people and f/5.6 for a group.
- Fashion. f/2.8 to f/5.6 on location. f/8 in the studio with flash, so the whole outfit is sharp.
- Wedding and events. f/2 to f/2.8 in dim halls. f/5.6 to f/8 for group photographs.
- Product. f/8 to f/11 for a sharp product. For more depth, stack several pictures instead of closing further.
- Automotive. f/8 to f/11 so the whole car is sharp. f/2.8 to f/4 for details such as a badge or a headlight.
- Photojournalism. f/5.6 to f/8 gives margin for focusing errors when events move fast.
- Sports and action. The widest aperture the lens offers, usually f/2.8 to f/4, for a fast shutter and a clean background.
- Landscape. f/8 to f/11 for sharpness from front to back. Use f/16 only when the foreground is very close, and watch for diffraction.
- Street and documentary. f/8 with zone focus in daylight. f/2 to f/4 in low light.
- Night and astro. The widest aperture, f/1.4 to f/2.8, to collect the most light.
- Cinema. The stop is chosen for the look, often T2.8 to T4. ND filters hold it there in bright light.
Questions beginners ask
Is f/1.8 or f/8 the bigger aperture?
f/1.8. The f-number is a fraction of the focal length, so a smaller number means a larger opening.
Why do f-numbers go 1.4, 2, 2.8, 4?
Each step halves the area of the opening, which needs the diameter divided by the square root of 2, about 1.4.
What is the difference between aperture and f-stop?
The aperture is the opening. The f-stop, or f-number, is how that opening is measured against the focal length.
What is a T-stop?
An f-number corrected for light lost inside the lens glass. It is always larger than the f-number.
Why does my phone not let me change aperture?
Many phone cameras have a fixed opening. The blur in portrait mode is usually added by software.
References
- Canon. (n.d.) EOS R6 Advanced User Guide: Av: Aperture-Priority AE. cam.start.canon/en/C004/manual/html/UG-02_ShootingMode_0070.html
- Edmund Optics. (n.d.) Limitations on Resolution and Contrast: the Airy Disk. edmundoptics.com/knowledge-center/application-notes/imaging/limitations-on-resolution-and-contrast-the-airy-disk
- International Organization for Standardization (2008) ISO 517:2008 Photography: Apertures and related properties pertaining to photographic lenses: Designations and measurements. 3rd edn. iso.org/standard/50089.html
- PetaPixel. (n.d.) F-Stops vs T-Stops: The Difference Explained in Plain English. petapixel.com/2016/12/30/f-stops-vs-t-stops-difference-explained-plain-english
- Präkel, D. (2009) Basics Photography 07: Exposure. Lausanne: AVA Publishing. See the section on stops (pp. 26–27).
- Toland, G. (1941) “Realism for Citizen Kane”, American Cinematographer, February 1941. theasc.com/article/realism-for-citizen-kane
How to cite this guide. Alex, A., & Joseph, A. (2026). Aperture: A guide for photographers and cinematographers. Creative Hut Academy. https://creativehut.org/library/what-is-aperture/
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