# What Is an F-Stop? The Ultimate Beginner's Guide to Aperture **By Chanuka Nayanajith** · 2026-09-23 **An f-number, often called an f-stop, is the number used to describe the aperture setting of a camera lens.** A lower f-number such as f/1.8 represents a wider aperture, while a higher f-number such as f/11 represents a narrower aperture. Changing the f-stop affects how much light passes through the lens and strongly influences depth of field. The numbers can seem backwards at first, but there is a logical reason for them. Once you understand what an f-number actually represents, settings such as f/2.8, f/5.6 and f/11 become much easier to use. ## What Is an F-Stop in Photography? The aperture is the adjustable opening inside a camera lens. The f-stop is the numerical setting used to describe that opening relative to the lens's focal length. In the standard optical definition, the f-number is calculated by dividing the focal length by the diameter of the entrance pupil, which is the effective aperture as viewed through the front of the lens. That is why the number is written as a ratio such as f/2, f/4 or f/8 rather than as a direct measurement in millimeters. For example, in the standard relationship, a 50mm lens at f/2 corresponds to an entrance pupil diameter of about 25mm. At f/4, it corresponds to about 12.5mm. The entrance pupil is not necessarily identical to the physical iris opening inside the lens because the lens elements in front of the aperture can change its apparent size. You do not need to calculate these measurements while taking photographs. The relationship is useful because it explains why lower f-numbers represent wider effective apertures. Nikon's [guide to aperture and f-stops](https://www.nikonusa.com/learn-and-explore/c/tips-and-techniques/understanding-maximum-aperture) also illustrates the relationship between f-number, light and depth of field. ## Why Does a Smaller F-Number Mean a Larger Aperture? The easiest way to understand the apparently reversed numbering is to remember that an f-number represents a ratio rather than the physical diameter of the opening. - **f/1.4, f/1.8 and f/2:** relatively wide apertures. - **f/4 and f/5.6:** moderate apertures. - **f/11, f/16 and f/22:** relatively narrow apertures. Therefore, moving from f/8 to f/4 opens the aperture rather than closes it. Moving from f/4 to f/8 does the opposite. A useful memory rule is simple: **lower f-number, wider opening; higher f-number, narrower opening.** ## F-Stop vs Aperture vs a Stop These terms are related, but they do not mean exactly the same thing. - **Aperture** is the opening in the lens through which light passes. - **F-number or f-stop** describes the aperture setting as a ratio, such as f/2.8 or f/8. - **A stop** is a relative way of describing a factor-of-two change in exposure or an equivalent change in resulting image brightness. With aperture and shutter speed, a full-stop change can double or halve the amount of light recorded. Digital ISO is different: changing ISO affects how the camera amplifies, processes or maps the captured signal rather than physically changing how much light reaches the sensor. In everyday photography, people often use “aperture” and “f-stop” almost interchangeably. Saying “set the aperture to f/4” and “set the f-stop to f/4” normally communicates the same practical instruction. The distinction becomes useful when learning exposure because the word “stop” is also used with shutter speed and ISO. The AAAPresets guide to [how aperture, shutter speed and ISO work together](/blogs/exposure-triangle/how-aperture-shutter-speed-and-iso-work-together) explains those relationships in more detail. ## How Full F-Stops Change the Amount of Light A common full-stop aperture sequence is: **f/1.4 → f/2 → f/2.8 → f/4 → f/5.6 → f/8 → f/11 → f/16 → f/22** Moving one full step toward a larger f-number reduces the light passing through the lens by approximately one stop. Moving one full step toward a smaller f-number increases it by approximately one stop. For example: - f/4 to f/5.6 allows approximately half as much light through the lens. - f/5.6 to f/4 allows approximately twice as much light through. - f/8 to f/11 is another one-stop reduction. - f/8 to f/5.6 is a one-stop increase. The numbers do not simply double because the amount of light depends on the area of the effective circular opening. Full f-numbers progress by approximately the square root of two, producing the familiar 1.4, 2, 2.8, 4 and 5.6 sequence. The displayed values are conventionally rounded, so practical light changes are described as approximate. Modern cameras often provide smaller adjustments as well. Many cameras use one-third-stop increments, while some models can also use half-stop increments. This is why you may see intermediate aperture settings such as f/3.2, f/3.5, f/4.5 or f/6.3. ## How F-Stops Affect Depth of Field Aperture does more than change the amount of light passing through the lens. It also strongly influences **depth of field**, which is the range of distances that appears acceptably sharp around the point of focus. When other important conditions remain similar: - A wider aperture such as f/1.8 or f/2.8 tends to create shallower depth of field. - A narrower aperture such as f/8 or f/11 tends to create greater depth of field. This is why photographers often use relatively wide apertures when they want a person, animal, product or detail to stand out against a softer background. However, the f-stop does not determine background blur by itself. Focal length, focusing distance and the distance between the subject and background also matter. In practical comparisons between different sensor formats, maintaining the same framing can require a change in focal length or shooting distance, which can also change the depth-of-field result. For example, moving closer to a subject or increasing the distance between the subject and background can produce stronger background separation even when the aperture remains unchanged. It is also useful to distinguish **background blur** from **bokeh**. Background blur describes the fact that an area is out of focus. Bokeh more specifically refers to the visual character or rendering of those out-of-focus areas. ## How F-Stops Work with Shutter Speed and ISO Changing aperture often means another exposure setting needs to change as well. Imagine you make a photograph at f/4, 1/250 second and ISO 100. If you change the aperture from f/4 to f/5.6, approximately one stop less light passes through the lens. If you want similar resulting brightness, one possible adjustment would be slowing the shutter speed from approximately 1/250 second to 1/125 second. Another option would be increasing ISO from 100 to 200. Those choices do not create identical photographs. The slower shutter speed gives subject and camera movement more time to appear in the image. Raising digital ISO changes how the captured signal is amplified or processed and can make noise more visible; it does not physically replace the light lost by closing the aperture. For a deeper explanation, see [what exposure means in photography](/blogs/photography-basics/what-is-exposure-in-photography-a-comprehensive-guide). ## What Does the Aperture Number on a Lens Mean? A lens name often includes its maximum aperture. Despite the word “maximum,” this is represented by the **lowest f-number** available because maximum aperture means the largest available opening. For example, a 50mm f/1.8 lens can normally open as wide as f/1.8. You can then stop it down to higher f-numbers such as f/2.8, f/4, f/5.6 or f/8 if the lens supports those settings. ### Constant-Aperture Zoom Lenses A lens described as 24-70mm f/2.8 has a maximum aperture of f/2.8 throughout its stated zoom range. Photographers commonly call this a constant-aperture zoom. ### Variable-Aperture Zoom Lenses A lens described as 18-55mm f/3.5-5.6 has a variable maximum aperture. It may open to approximately f/3.5 at the wide end, while its widest available aperture becomes approximately f/5.6 toward the long end. The exact behavior depends on the lens, so check its specifications rather than assuming every zoom lens handles aperture in the same way. ### Maximum vs Minimum Aperture The terminology can initially sound confusing: - **Maximum aperture:** the widest opening and usually the lowest available f-number. - **Minimum aperture:** the narrowest opening and usually the highest available f-number. ## Which F-Stop Should You Use? There is no single correct f-stop for a photography genre. Choose aperture according to the depth of field, available light, subject movement, lens and visual result you need. ### Portraits A relatively wide aperture such as f/1.8, f/2 or f/2.8 can help separate a single subject from the background. You do not have to use the lens completely wide open. If depth of field becomes too shallow to keep the important features acceptably sharp, try f/2.8, f/4 or another narrower aperture. Focal length and subject-to-background distance can be just as important as aperture when creating separation. ### Groups of People Groups often require more depth of field than a single-person portrait. A setting around f/4, f/5.6 or f/8 can be a useful starting range, but the appropriate choice depends on how many rows of people you have, their distance from the camera, focal length and where you focus. ### Landscapes For many landscapes, settings around f/5.6 to f/11 can provide a useful balance between depth of field and image detail. That is a starting point, not a rule. Focus distance, focal length and the distance between foreground and background subjects also matter. Do not automatically choose f/22 simply because you want everything sharp. ### Low-Light Photography A wide aperture can be useful when light is limited because it lets more light pass through the lens and may allow a faster shutter speed or lower ISO. The trade-off is depth of field. Opening from f/4 to f/1.8 may help in low light, but the shallower focus range might not suit a group photograph or a subject moving toward and away from the camera. ### Macro and Close-Up Photography Depth of field can become extremely shallow at close focusing distances, even when you stop the lens down. Macro photographers may therefore use narrower apertures than portrait photographers, but very high f-numbers can introduce additional diffraction softness. ## Why f/16 or f/22 Does Not Automatically Make Everything Sharper Stopping down generally increases depth of field, but greater depth of field is not the same thing as greater optical sharpness. At very narrow apertures, diffraction becomes increasingly important. Light spreads as it passes through the small aperture, which can reduce the finest resolved detail. The visible effect depends on the camera, lens, aperture, output size and how closely the image is viewed. This does not mean you should never use f/16 or f/22. Those settings can be completely reasonable when the additional depth of field, slower shutter speed or a particular optical effect is more important than maximum fine-detail resolution. The practical lesson is to choose the aperture your photograph requires rather than assuming the largest available f-number will always create the sharpest result. ## How to Practise F-Stops with Aperture Priority Aperture Priority mode is one of the easiest ways to see what f-stops do. Depending on the camera manufacturer, the mode is commonly labelled **A** or **Av**. You choose the aperture, while the camera typically adjusts shutter speed according to its exposure calculation. ISO may remain fixed at the value you selected or change automatically when Auto ISO is enabled, depending on the camera and configuration. Sony's [aperture and A-mode guide](https://www.sony.com/electronics/support/articles/00267926) provides one manufacturer example. 1. Choose a stationary subject with objects both in front of and behind it. 2. Keep the camera position and focal length consistent. 3. Focus on the same part of the subject for every photograph. 4. Take one photograph near your lens's widest available aperture. 5. Repeat at f/4, f/5.6, f/8 and f/11 where your lens permits. 6. Compare the foreground, subject and background at a larger viewing size. 7. Also compare the shutter speed and ISO recorded for each photograph. This exercise makes the relationship between aperture, depth of field and available light much easier to understand than memorizing a list of numbers. If you are still learning the other controls on your camera, the AAAPresets [beginner photography guide](/blogs/photography-basics/photography-for-beginners-everything-you-need-to-know-to-get-started) provides the broader context. ## Common F-Stop Mistakes - **Assuming a low f-number is a small opening:** the relationship is reversed; a lower f-number means a wider aperture. - **Using the widest aperture for every portrait:** very shallow depth of field can leave important details outside the acceptable focus range. - **Assuming f/8 or f/11 is universally the sharpest setting:** lens performance varies, and depth-of-field requirements may matter more than a theoretical sweet spot. - **Automatically choosing f/22 for landscapes:** a narrow aperture increases depth of field but can also increase diffraction softness. - **Thinking aperture alone controls background blur:** focal length, focusing distance and subject-to-background distance also matter. - **Changing aperture without watching shutter speed:** stopping down in limited light can result in a shutter speed slow enough to introduce unwanted camera or subject movement. - **Confusing bokeh with depth of field:** depth of field describes the range that appears acceptably sharp, while bokeh describes the character of out-of-focus rendering. ## The Simple Way to Remember F-Stops If you remember only three things, make them these: - **Lower f-number = wider aperture = more light through the lens.** - **Higher f-number = narrower aperture = less light through the lens.** - **Wider apertures generally reduce depth of field, while narrower apertures generally increase it.** Everything else becomes easier once those relationships make sense. Instead of searching for one “perfect” f-stop, decide how much of the photograph needs to appear sharp, make sure your shutter speed remains suitable for the subject, and adjust the other exposure settings as necessary. Written by [Asanka Dilshan](/pages/about-us), founder of AAAPresets. **Tags:** Aperture, Beginner's Guide, Camera Settings, Exposure Triangle, F-Stop, Guide, Photography Basics --- > Source: [aaapresets](https://aaapresets.com/blogs/aperture/what-is-an-f-stop-the-ultimate-beginners-guide-to-aperture)