How the F-Stop Works

By: Nathan Chandler  | 
A camera lens positions to highlight the f-stop settings. 
Like a little mumbo jumbo for lunch? Actually, f-stops and focal lengths aren't all that bewildering once you know what the numbers stand for. See more camera stuff pictures.

Photography deals with capturing light in a way that appeals to your artistic sensibilities, whether you prefer perfectly-lit, tack-sharp portraits worthy of presidential candidates or blurrier, artistic renderings of NASCAR competitors roaring by at 180 mph (290 kph). To consistently create the kinds of pictures that will make your creative spirit soar, you need a firm grasp of common camera settings such as shutter speed, ISO setting and f-stop (or focal stop).

F-stops in particular have a tremendous effect on image characteristics, some of which may not be obvious to amateur shooters. For the bulk of this article, we'll discuss and improve your grasp of the mysterious f-stop.


The other two points of what's known as the exposure triangle are shutter speed and ISO [source: Hall]. These are also important concepts that will help you flesh out your understanding of how cameras work. For more information on these subjects, check out our articles "What is ISO speed?" and "10 Important Photography Terms."

F-Stop Basics

When you take pictures, you're using the interplay between shutter speed, ISO and f-stop settings to control exposure. F-stops especially seem to baffle many novice shooters, perhaps because of the weird alphanumeric symbols used to indicate these settings. Happily, figuring out f-stops is really pretty simple once you disassemble the term and break it up into understandable parts.

For starters, your camera has a mechanical aperture that controls how much light enters the camera. This aperture can change in size, and it works a lot like the pupil in your eye. In general, the brighter the scene, the more the pupil constricts; in low light, the pupil is larger, letting in as much light as possible. The same goes for your camera's aperture in most situations.


The f-stop number is determined by the focal length of the lens divided by the diameter of the aperture. A lens focal length refers to a lens' field of view (sometimes called angle of view), which is the width and height of the area that a particular lens can capture. Focal length is often printed right on the camera lens.

A lens with a 100mm focal length set to an f-stop of f/10 has an aperture diameter of 10mm. Keep in mind that doubling the f-stop number halves the size of the aperture opening. So, moving from f/10 to f/20 decreases the size of the aperture from 10mm to 5mm. If you use an SLR (single-lens reflex) camera that lets you affix a range of lenses, the f-stop numbers will be different for each lens.

All technical details aside, you won't need to spend a lot of time sweating the finer points of f-stops. What you really need to know is how use your camera's aperture scale to your advantage as you shoot. Keep reading to find out how aperture control affects the way your pictures look.


Tips for How to Set the F-Stop

A close-up photograph of a tulip. 
Comstock Images/┬ęGetty Images/Thinkstock

When you want to take sharply focused pictures, your camera needs as much light as possible. So, in dimly lit areas, it's best to choose a low f-stop number, opening the aperture to its biggest size to let in more light.

You can use your camera's manual mode (M) or aperture priority mode (AV, or aperture value) to take full control of your camera's aperture [source: Cincotta]. In addition to controlling how much light enters the camera, changing the size of the aperture also changes image depth.


As you tweak your camera's aperture, you're altering the lens' depth of field. Depth of field is another photography concept that's easily clouded by complicated mathematics and esoteric language, but, basically, it refers to how much of a scene is in focus.

When subjects both near and far are relatively crisp and sharp, many photographers say a scene has deep depth of field. Shallow depth of field indicates that only part of a scene is in sharp focus.

Shallow depth of field is a powerful tool for making great pictures by drawing attention to specific aspects of a picture. For example, if you compose a portrait in which the subject's eyes are the only facial feature in focus, you're isolating the eyes and making them stand out in an arresting way that your viewers can't miss.

Lenses that boast a large maximum aperture, such as f/1.2 or f1/4, are best for creating extremely shallow depth of field. To accentuate this effect, it helps to be close to your subject [source: Golowczynski].

The reverse is true if you want deep depth of field. Many landscape photographers use high f-stops with small apertures in the range of f/16 or f/22, which helps keep objects in both the foreground and background in focus.

It's worth noting that the image quality of many lenses tends to deteriorate as you approach the extreme ends of the f-stop range. This is especially true of the telephoto lens, because they're often so complex.

The fastest way to understand how to make f-stops work for you is to experiment. Pick one subject and shoot it using different f-stop settings. Review the images to see how sharpness and brightness change from image to image. Regardless of the kinds of subjects you choose to photograph, understanding f-stops, aperture and depth of field can help you make a mundane scene totally marvellous.


The Power of F-Stop Mastery

Taking the time to understand advanced photography concepts like aperture range, f-numbers, and field depth go a long way in boosting your ability to take better pictures. By doing so, you'll unlock the ability to use light and focus to your advantage.

Again, it's important to practice adjusting f-stops in various lighting conditions and with different subjects to see firsthand how these settings impact your photographs. Whether you're a beginner learning the basics or a seasoned pro refining your technique, mastery of the f-stop will most certainly enhance the quality and expressiveness of your images.


Lots More Information

Related Articles

  • Cambridge in Colour. "Camera Exposure." (Feb. 6, 2011)
  • Cambridge in Colour. "Understanding Camera Lenses." (Feb. 6, 2011)
  • Caponigro, John Paul. "Testing for the Ideal Lens Aperture." (Feb. 6, 2011)
  • Clay, Willard. "Control Your Depth of Field." Aug. 31, 2010. (Feb. 6, 2011)
  • Nikon USA. "Photography Glossary." (Feb. 6, 2011)
  • Nikon USA. "Digital SLR Camera Basics." (Feb. 6, 2011)
  • PC Mag. "Definition of f-stop." (Feb. 6, 2011),2542,t=f-stop&i=57755,00.asp
  • Peterson, Bryan F. "Basic Photo Tips: Aperture, Shutter Speed, and ISO." August 2008. (Feb. 6, 2011)
  • Sheppard, Rob. "Isolated Sharpness." May 25, 2010. (Feb. 6, 2011).