How the F-Stop Works

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.

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.

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.